Fig. 153.—Alternations of basalt and tuff with shale, etc., Kingswood Craig, Burntisland.
Where the phenomena of the puys have been most typically developed, lavas and tuffs succeed each other in rapid succession, with numerous or occasional interstratifications of ordinary sediment. Perhaps the most complete and interesting example of this association is to be found on the coast between Burntisland and Kirkcaldy, where, out of a total thickness of rock which may be computed to be between 1500 and 2000 feet, it will probably be a fair estimate to say that the igneous materials constitute four-fifths, or from 1200 to 1600 feet. The lavas are varieties of basalt ranging in character from a black compact columnar to a dirty green earthy cellular or slaggy rock. Each separate flow may be on the average about 20 or 30 feet in thickness. Columnar and amorphous sheets succeed each other without any interposition of fragmentary material ([Fig. 171]). But along the junctions of the separate flows layers of red clay, like the bole between the basalts of the Giant's Causeway, may frequently be noticed. The characteristic slaggy aspect of the upper parts of these ancient coulées is sometimes remarkably striking. The full details of this most interesting section will be given in later pages ([p. 470]). But some of its more characteristic external features may be understood from the views which are presented in Figs. [152], [153], [170], [171].
The general bedded character of the volcanic series is well shown in [Fig. 153], which represents the alternations of lavas and tuffs in the Kingswood Craig two miles to the east of Burntisland. The harder basalts will be seen to project as bold crags while the tuffs and other stratified deposits between them give rise to grassy slopes and hollows. A nearer view of the alternation of lavas and tuffs with non-volcanic sedimentary deposits is supplied in [Fig. 170], which is taken from a part of the Fife coast a little further to the east than the last illustration. Here one of the limestones of the Carboniferous Limestone series is overlain with shale and tuff, which, being easily disintegrated, have been cut away by the waves, leaving the lava above to overhang and fall off in blocks. The columnar structure of some of the basalts of this coast is well brought out in [Fig. 171], which shows further how the columns sometimes merge into an amorphous part of the same sheet.
These Fife basalts illustrate admirably the peculiarities of the sheets of lava which are intercalated among the Carboniferous strata. They show how easy it generally is to discriminate between such sheets and intrusive sills. The true lavas are never so largely crystalline, nor spread out in such thick sheets as the sills; they are frequently slaggy and amygdaloidal, especially towards the top and bottom, the central portion being generally more fine-grained and sometimes porphyritic. Where most highly cellular they often decompose into a dull, earthy, dirty-green rock. Where they form a thick mass they are usually composed of different beds of varying texture. Except the differences between the more compact centre and the slaggy layer above and below, the bedded lavas do not present any marked variation in composition or structure within the same sheet. A striking exception to this rule, however, is furnished by the Bathgate "leckstone" already described.[461] This mass forms a continuation of the great basaltic ridge of the Bathgate Hills, and though its exact relations to the surrounding strata are concealed, it appears to be an interbedded and not an intrusive sheet. The remarkable separation of its constituent minerals into an upper, lighter felspathic layer, and a lower, heavier layer, rich in olivine, augite and iron-ores, is a structure which might be more naturally expected to occur in a sill. An instance of its development in an undoubted sill will be described further on. Nevertheless, if we follow the trend of the volcanic band of the Bathgate Hills southward for only two miles beyond the picrite quarry, we find in the Skolie Burn a rock in many respects similar, and quarried for the same purpose of building oven-soles. This "leckstone" is there seen to be surmounted by a group of calcareous shales and thin limestones. The section laid bare in the stream is represented in [Fig. 154]. Immediately above the diabase, which is highly cellular, lies a green felspathic sandstone or shale containing detached fragments of the amygdaloid together with Lingulæ and other shells. There seems no reason to doubt that this is a true interstratified lava.[462]
[461] Trans. Roy. Soc. Edin. xxix. (1879) p. 504.
[462] Trans. Roy. Soc. Edin. xxix. (1879), pp. 505-507.
Fig. 154.—Section of the upper surface of a diabase ("leckstone") sheet, Skolie Burn, south-east of Bathgate.
1. Slaggy diabase; 2. Green sandy shale and shaly sandstone containing Lingulæ, also pieces of slag from the underlying lava, which are completely wrapped round in the sediment; 3. Yellow calcareous shelly sandstone; 4. Dark shale with Spiriferæ, etc.; 5. Bed of blue crinoidal limestone; 6. Clays and thin coal; 7. Black and blue calcareous shales and thin limestones.
Where the puys attained their greatest development in Scotland, they rose in the shallow lagoons, and here and there from deeper parts of the sea-bottom, until by their successive discharges of lavas and tuffs they gradually built up piles of material, which, in the Linlithgow and Bathgate district, may have been nearly 2000 feet in thickness. It must be remembered, however, that the eruptions took place in a subsiding area, and that even the thickest volcanic ejections, if the downward movement kept pace with the volcanic activity, need not have grown into a lofty volcanic hill. Indeed, largely as the lavas and tuffs bulk in the geology of some parts of Central Scotland, their eruption does not seem to have seriously interfered with the broader physical changes that were in progress over the whole region. Thus the subsidence which led to the spread of a marine and limestone-making fauna over much of Central Scotland included also the volcanic districts. The limestones, formed of crinoids, corals and other marine organisms, extended over the submerged lavas and tuffs, and were even interstratified with them.