The Ancient Volcanoes of Great Britain, Volume 2 (of 2)

by Archibald Geikie

With the important exception of the Snowdonian region and possibly others, we find that the earlier eruptions of each period were generally most basic, and that the later intrusions were most acid. Thus the diabase-lavas and tuffs at the base of the Cambrian series of St. David's are pierced by quartz-porphyry veins. The andesites of the Lower Old Red Sandstone were succeeded by bosses, sills, and dykes of granite, felsite, and lamprophyre. The eruptions of the Carboniferous plateaux began with extremely basic lavas, and ended with trachytes, felsites, and quartz-porphyries. The basalts of the great lava-fields of the Tertiary period are pierced by masses of granophyre and even granite.

There has evidently been, on the whole, a progressive diminution in the quantity of bases and a corresponding increase in the proportion of acid in the lavas erupted during the lapse of one volcanic period. This sequence is so well marked and so common that it cannot be merely accidental. The acid and basic rocks, occurring as they do at each volcanic centre in the same relation to each other, are obviously parts of one connected series of eruptions. We seem to see in this sequence an indication of what was taking place within the subterranean magma. There was first an extensive separation of the more basic constituents, such as the ferro-magnesian minerals and ores, and the lavas which came off at that time were heavy and basic basalts, and even picrites. The removal of these elements left the magma more acid, and such rocks as andesites were poured out, until at last the deeper intrusive sills, dykes and bosses became thoroughly acid rocks, such as felsite, quartz-porphyry and granite, while if any superficial outflow took place it was such a rock as dacite.

In the case of the Tertiary volcanic series there is evidence that after the acid protrusions a final uprise of basic material occurred. No satisfactory proof of any similar return to basic eruptions has been detected among the Palæozoic formations. But it is possible that some of the basic sills and dykes, the precise age of which cannot be fixed, may really mark such a reversion, even in the earlier volcanic periods.

Some illustrative examples of volcanic cycles from other countries were cited in Chapter iii. To these I may add another instance which presents a close analogy to some of the phenomena characteristic of the British examples of Palæozoic as well as of Tertiary age. Monte Venda in the Euganean Hills, already alluded to (p. 474), may be cited as an interesting specimen of an older Tertiary volcano, which has been so dissected by denudation as to show not only the succession of its superficial discharges, but the position and order of its subterranean intrusions. The volcanic eruptions of this neighbourhood, judging from the area which they still cover and the height they reach, may have piled up a mountain rivalling or surpassing Etna in dimensions. In Monte Venda the lowest visible igneous rocks are sills of oligoclase-trachyte that have been thrust between and have highly altered Cretaceous (Tithonian) limestones. Other intrusive sheets of trachyte follow in the overlying Cretaceous strata (Neocomian and Scaglia). It is not until the older Tertiary formations are reached that undoubted tuffs and lavas occur, indicative of truly interstratified volcanic materials. These formations, consisting of nummulitic limestones and other strata together with fossiliferous tuffs, show that the volcano began as a submarine vent. It discharged dark basic dolerites and tuffs. The highest lava, however, crowning the summit of the mountain is a trachyte. There appears to have been a rapid decrease of the bases in the magma, for the later lavas were rhyolites, accompanied with rhyolitic tuffs of Oligocene age, and followed in the end by the black vitreous trachyte of Monte Sieva.

12. From the evidence detailed in these volumes, it appears that the sequence from basic to acid discharges was on the whole characteristic of each eruptive period. It is obvious, however, that as the protrusions of successive periods took place within the same limited geographical area, the internal magma during the interval between two such periods must in some way have been renewed as regards its constitution, for when, after long quiescence, eruptions began once more, basic lavas appeared first and were eventually followed by acid kinds. This cycle of transformation is admirably exhibited in Central Scotland, where the andesites of the Old Red Sandstone with their felsite sills are followed by the limburgites, picrites and other highly basic lavas at the bottom of the Carboniferous plateaux, succeeded in turn by the andesites, trachytes and acid sills of that series. When the puy eruptions ensued, the magma had once more become decidedly basic.

That the true explanation of these alterations is of a complex order may be inferred from the exceptions which occur to the general rule. I have alluded to the Snowdon region, where the acid rhyolites are followed by more basic andesites, and where the sills are also more basic than the superficial lavas. In the Arenig and Cader Idris country the sills are likewise more basic than the bedded lavas. Among the Carboniferous puys of the basin of the Firth of Forth, the sills are not sensibly more acid than many of the superficial basalts, and they even include such rocks as picrite. Possibly in this last-named region we see an arrested sequence, the volcanic protrusions having from some cause ceased before the general uprise of the more acid magma.

INDEX

[[A]] [[B]] [[C]] [[D]] [[E]] [[F]] [[G]] [[H]] [[I]] [[J]] [[K]] [[L]] [[M]] [[N]]
[[O]] [[P]] [[R]] [[S]] [[T]] [[U]] [[V]] [[W]] [[Y]]

AA form of lava in the Sandwich Islands, ii. [187]
Abereiddy Bay, i. [206]
Abich, H., i. [32]
Acid igneous rocks, silica percentage of, i. [14];
devitrification of, [19];
flow-structure of, [21];
occur in thicker sheets than basic, [24];
alternations of, with basic, [28], [61], [152], [157], [165], [207], [213], [233], [284], [318]; ii. [236], [266], [278];
metamorphic action of, i. [95], [96];
connection with mountains, ii. [98];
scenery of, [102].
Acids, mineral, at volcanoes, i. [72]
Acland, Mr. H. D., i. [133]
Aegean Sea, volcanoes of, i. [1]
Agglomerates, i. [31], [57], [58];
in dykes, [70];
Archæan, [120], [130], [135];
Cambrian, [148], [149], [167];
Silurian, [178], [180], [181], [184], [185], [194], [199], [206], [214], [237], [241], [244], [247], [253], [255];
Old Red Sandstone, [279], [285], [289], [300], [313], [325], [338], [349], [352];
Carboniferous, [381], [399], [402], [404], [427], [429], [439], [440]; ii. [13], [24], [28], [29];
Permian, [62], [64], [99];
Tertiary, [194], [277], [278], [281], [289], [292], [293], [384], [400], [423]
Allan, T., i. [363]
Allotriomorphic minerals, i. [21]
Allport, Mr., i. [95], [130], [131], [260], [451]; [ii]. [11], [42], [102], [103], [104], [106], [370]
Amber in Tertiary volcanic series, ii. [198]
America, Western North, volcanic rocks of, i. [10], [100]; ii, [267]
Amygdales, origin of, i. [15]; ii. [189], [221], [285], [290]
Amygdaloidal structure, i. [15], [16], [17], [59], [274], [385]; ii. [3], [31], [57], [129], [188]
Analyses of Cambrian tuffs, i. [148], [149];
of Cambrian diabases, [153];
of Old Red Sandstone diabases, [274];
of Old Red Sandstone andesites, [275];
of Old Red Sandstone trachytes, [276];
of Old Red Sandstone felsites, [278];
of Carboniferous limburgite, [377];
of Carboniferous basalts, [379];
of Carboniferous trachytes, [380];
of Carboniferous phonolite, [381];
of Tertiary trachyte, ii. [139];
of Tertiary dacite, [244]
Anderson, Dr. Tempest, ii [261], [262], [263]
Andesite, i. [24], [131], [136], [164], [165], [167], [178], [180], [184], [189], [190], [204], [212], [213], [214], [215], [229], [230], [245], [246], [247], [252], [274], [275], (analyses), [277], [292], [300], [306], [309], [315], [318], [325], [330], [333], [345], [377], [379], [386], [403], [421]; ii. [45], [57], [96], [125], [137], [184], [236], [424]
Anglesey, gneisses and schists of, i. [126];
volcanic rocks of, [189], [219]
Anhydrite deposits, ii. [54]
Annandale, Permian volcanic rocks of, ii. [56], [58], [60], [61], [66]
Antrim, Old Red Sandstone volcanic rocks of, i. [314];
Tertiary volcanic rocks of, [47], [52]; ii. [109], [110], [113], [139], [140], [199];
basalts of, [192], [193], [199], [202], [206];
clays and iron-ore of, [204];
rhyolites of, [185], [364], [370], [371], [426], [445];
deceptive agglomerate of, [188];
rhyolitic conglomerate of, [195], [206];
plateau of, [199];
tuffs of, [202], [204];
vents of, [271], [277];
sills of, [298];
central subsidence of basalt-plateau of, [448]
Apatite, ii. [135]
Apjohn, J., [ii]. [42]
Applecross, volcanic vents in, ii. [292]
Arans, the, i. [175], [176], [179], [184], [186], [207]
Archæan period, i. [110], [111];
volcanic rocks of, [120]
Ardnamurchan, dykes and veins of, ii. [154], [320];
basalt-plateau of, [208];
vents of, [287];
sills of, [318];
gabbro of, [355]
Arenig group, i. [175];
lower limit of, [177], [185];
top of, [178], [228], [246]
—— volcano of, i. [42], [175], [176], [179], [186], [207]
—— rocks in Scottish Highlands, i. [123], [126];
in Merionethshire, [176], [179];
of Shropshire, [189];
of Ayrshire, [196];
of Scottish Highlands, [201];
of Anglesey, [221];
of Lake district, [229];
of Ireland, [239]
Argyll, Duke of, ii. [113], [114], [198]
Argyllshire, dykes of, ii. [127], [128], [138], [142], [146], [171], [172];
vents of, [278]
Arizona, explosion crater in, i. [58];
laccolites in, [86]
Arran, Old Red Sandstone volcanic rocks of, i. [298], [311];
Carboniferous volcanic rocks of, [386], [392];
possible Permian volcanic rocks of, ii. [58];
granite of, i. [93]; ii. [366], [367], [418];
pitchstone of, i. [19]; ii. [445];
dykes of, [123], [139], [140], [142], [146], [154], [161]
Arthur Seat, i. [364], [373], [378], [385], [386]; ii. [67]
"Arvonian," i. [145], [156]
Asbestos in volcanic breccia, ii. [51]
Ascension Island, cellular lava of, i. [15]
Ashes, volcanic (see [Tuffs])
Ashprington volcanic series, i. [262]
Asphalt, ii. [79]
Atherstone, i. [170]
Augite, loose crystals of, in volcanic vents, i. [62], [178], [181]; ii. [58], [79];
lumps of, in volcanic vents, i. [352]
Augite-aphanites, i. [178]
Auvergne, old volcanoes of, i. [29], [32], [66], [70], [100]; ii. [373]

Aveline, Mr. W. T., i. [227], [230]; ii. [32]
Ayrshire, example of volcanic neck in, i. [56];
Silurian volcanic rocks of, [192];
Old Red Sandstone volcanic rocks of, [275], [282], [283], [285], [291], [331];
Carboniferous volcanic plateau of, [102], [368], [388], [393], [398], [410];
Carboniferous Puys of, [415], [416], [434], [440], [474];
Permian volcanic rocks of, ii. [55], [58], [62]
Azoic period, i. [109]
Bäckström, Mr., ii. [266]
Baily, W. H., i. [251], [252]; ii. [198], [449]
Bala group, i. [175], [190], [196], [201], [206], [207], [223], [242];
limestone of, [47], [175], [229], [245], [251];
volcanic rocks of, [186], [190], [207], [213], [221], [241], [248]
Balbriggan, igneous rocks of, i. [244]
Ballagan beds (Lower Carboniferous), i. [384], [387], [392], [393], [412], [447]
Ballantrae, volcanic rocks at, i. [192], [199]
Ballypallidy, tuffs and leaf-beds of, ii. [204], [429]
Bamborough, Whin Sill at, ii. [2], [3], [5]
Banding of igneous rocks, i. [84], [207]; ii. [189], [294], [329], [354], [357], [476]
—— of gneiss, i. [116]
Bangor group, i. [166]
Banks, Sir Joseph, ii. [109]
Barnavave, eruptive rocks of, ii. [421]
Barrow, Mr. G., i. [201], [226], [272], [279], [380]; ii. [147], [148]
Basalt, columnar structure of, i. [24], [25];
relation to gabbro, [78];
altered by carbonaceous strata, [95];
shells supposed to occur in, ii. [110];
banded, [189];
thickness of sheets of, [192];
meaning of red layer between sheets of, [197], [203], [206], [254];
metamorphism of, [272], [276], [337], [339], [340], [347], [355], [356], [357], [358], [362], [378], [383], [386], [397], [399], [400], [404], [413]
—— pre-Cambrian, i. [119], [131];
Silurian, [206], [207], [230], [245];
Carboniferous, [378], [403], [407], [417]; ii. [11], [45], [46];
Permian, [57], [96];
Tertiary, [125], [136], [183], [199], [208], [291]
Basalt-conglomerate, ii. [195]
Basic volcanic rocks, silica-percentage of, i. [14];
devitrification of, [20];
flow-structure of, [21];
occur in thinner sheets than the acid, [24];
metamorphic action of, [94];
erupted at low levels, [98];
scenery of, [102];
converted into schists by deformation, [75], [114], [118], [119], [124], [129];
alternation with acid, [28], [61], [131], [157], [165], [207], [213], [233], [284], [318]; ii. [236], [266], [278]
Bass Rock, i. [372], [373], [403]
Bassenthwaite Lake, i. [335]
Bathgate, puy eruptions of, i. [440], [442], [445], [456], [461]
Bauer, Dr. M., i. [62]
Bauxite, ii. [197], [204]
Bayley, Mr. W. S., ii. [330]
Bedding in lavas, i. [24]
Bell, Sir I. Lowthian, ii. [1], [113], [137], [165]
Bemrose, Mr. H. A., ii. [10], [11], [13], [16], [17], [18], [20], [21]
Ben Cruachan, alteration of granite at, i. [343]
—— Hiant, basic sills of, ii. [318]
Benaun More, felsite of, i. [347]
Berger, J. F., ii. [22], [95], [110], [113], [139], [140], [141], [145], [199], [364], [426]
Bertrand, Prof. M., i. [28]
Berwickshire, i. [272], [290], [338], [375], [385], [401], [413]
Berwyn Hills, i. [176], [186], [208], [218]
Biggar, volcanic area, i. [287], [325]
Binney, E., ii. [56]
Binny Craig type of basalt, i. [419], [421] (444)
Biotite (see [Mica])
Bitumen in intrusive rocks, i. [421]
Blackstone (Derbyshire), ii. [18], [21]
Blair-Atholl Limestone, i. [122]
Blake, Rev. J. F., i. [126], [130], [144], [160], [161], [162], [163], [165], [166], [168], [220], [221], [222]
Blocks, ejected, i. [36], [423], [438]; ii. [197], [221]
Bole between lavas, i. 442; ii. [197], [203], [206], [254]
Bombay, volcanic plateau of, ii. [180]
Bombs, volcanic, i. [60]; ii. [39]
Bonney, Prof., i. [95], [126], [130], [136], [144], [160], [162], [163], [164], [165], [166], [167], [168], [192], [210], [227]
Borrowdale Volcanic Series, i. [227]
Bosses, volcanic, i. [56], [78], [88];
petrography of, [89];
differentiation in, [90]; ii. [476];
granitic, i. [93];
metamorphism around, [94], [95];
conditions of their intrusion, [97], [98];
weathering of, [102]
—— Silurian, i. [215], [235];
Old Red Sandstone, [277], [288];
Carboniferous, [403], [458]
—— Tertiary, ii. [271], [284], [327], [366], [378], [395], [403];
boundaries of, [382];
relation to older eruptive vents, [280], [384], [399];
relation to plateau basalts, [386], [396], [402], [404];
relation to gabbro intrusions, [391], [402], [404];
relation to the basic dykes, [395]
Bostonite, ii. [47]
Boué, Ami, i. [268], [363]; ii. [112], [372]
Boule, M., i. [27], [29], [44], [45], [46], [61]; ii. [375]
Boutan, M., i. [62]
Bowden Hill, type of doleritic basalt, i. [418], [421]
Braid Hills, great vent of, i. [289], [293], [311], [318], [323]
Branco, Prof. W., i. [46], [417]
Breccias, volcanic, i. [31], [32], [120], [131], [135], [147], [165], [189], [190], [197], [213], [224], [225], [233], [234], [246], [252], [255], [289], [347]; ii. [39], [41], [49], [195]
—— of non-volcanic materials, ii. [196], [423]
Brecciated structure, i. [162], [211]
Breidden Hills, i. [176], [190], [208]
Brent Tor, ii. [33], [35], [36]
Bréon, M. R., ii. [191]
Britain, advantageous position of, for the study of ancient volcanic action, i. [6];
completeness of the Geological Record in, [6];
direction of folds and fractures in, [11];
chief lavas found in, [31];
Vesuvian cones of, [42];
volcanic plateaux of, [43];
puys of, [46];
lacustrine volcanoes of, [49];
fissure eruptions of, [52];
scenery of volcanic rocks of, [100], [101];
pre-Cambrian rocks of, [111];
in Cambrian time, [141];
in Silurian time, [173];
in Devonian time, [258];
in Old Red Sandstone time, [263];
in Carboniferous time, [355];
in Permian time, ii. [53];
in older Tertiary time, [108]
Brögger, Prof., i. [28], [88], [90], [91], [92]
Bryce, J., i. [314], [369]
Buch, L. von, i. [27]; ii. [381]
Buckland, W., ii. [95], [110], [113]
Buddle, J., ii. [113]
Builth, i. [176], [203]
Burdiehouse Limestone, i. [361], [374], [388], [415], [463]
Burnt Country of Asia Minor, i. [2]