Some geologists had erroneously supposed, from observations made on recent cones of eruption, that lava which consolidates on steep slopes is always of a scoriaceous or vesicular structure, and never of that compact texture which we find in those rocks which are usually termed “trappean.” Misled by this theory, they have gone so far as to believe that if melted matter has originally descended a slope at an angle exceeding four or five degrees, it never, on cooling, acquires a stony compact texture. Consequently, whenever they found in a volcanic mountain sheets of stony materials inclined at angles of from 5° to 20° or even more than 30°, they thought themselves warranted in assuming that such rocks had been originally horizontal, or very slightly inclined, and had acquired their high inclination by subsequent upheaval. To such dome-shaped mountains with a cavity in the middle, and with the inclined beds having what was called a quâquâversal dip or a slope outward on all sides, they gave the name of “Elevation craters.”
As the late Leopold Von Buch, the author of this theory, had selected the Isle of Palma, one of the Canaries, as a typical illustration of this form of volcanic mountain, I visited that island in 1854, in company with my friend Mr. Hartung, and I satisfied myself that it owes its origin to a series of eruptions of the same nature as those which formed the minor cones, already alluded to. In some of the more ancient or Miocene volcanic mountains, such as Mont Dor and Cantal in central France, the mode of origin by upheaval as above described is attributed to those dome-shaped masses, whether they possess or not a great central cavity, as in Palma. Where this cavity is present, it has probably been due to one or more great explosions similar to that which destroyed a great part of ancient Vesuvius in the time of Pliny. Similar paroxysmal catastrophes have caused in historical times the truncation on a grand scale of some large cones in Java and elsewhere.[[1]]
Among the objections which may be considered as fatal to Von Buch’s doctrine of upheaval in these cases, I may state that a series of volcanic formations extending over an area six or seven miles in its shortest diameter, as in Palma, could not be accumulated in the form of lavas, tuffs, and volcanic breccias or agglomerates without producing a mountain as lofty as that which they now constitute. But assuming that they were first horizontal, and then lifted up by a force acting most powerfully in the centre and tilting the beds on all sides, a central crater having been formed by explosion or by a chasm opening in the middle, where the continuity of the rocks was interrupted, we should have a right to expect that the chief ravines or valleys would open towards the central cavity, instead of which the rim of the great crater in Palma and other similar ancient volcanoes is entire for more than three parts of the whole circumference.
If dikes are seen in the precipices surrounding such craters or central cavities, they certainly imply rents which were filled up with liquid matter. But none of the dislocations producing such rents can have belonged to the supposed period of terminal and paroxysmal upheaval, for had a great central crater been already formed before they originated, or at the time when they took place, the melted matter, instead of filling the narrow vents, would have flowed down into the bottom of the cavity, and would have obliterated it to a certain extent. Making due allowance for the quantity of matter removed by subaërial denudation in volcanic mountains of high antiquity, and for the grand explosions which are known to have caused truncation in active volcanoes, there is no reason for calling in the violent hypothesis of elevation craters to explain the structure of such mountains as Teneriffe, the Grand Canary, Palma, or those of central France, Etna, or Vesuvius, all of which I have examined. With regard to Etna, I have shown, from observations made by me in 1857, that modern lavas, several of them of known date, have formed continuous beds of compact stone even on slopes of 15, 36, and 38 degrees, and, in the case of the lava of 1852, more than 40 degrees. The thickness of these tabular layers varies from 1½ foot to 26 feet. And their planes of stratification are parallel to those of the overlying and underlying scoriæ which form part of the same currents.[[2]]
Nomenclature of Trappean Rocks.—When geologists first began to examine attentively the structure of the northern and western parts of Europe, they were almost entirely ignorant of the phenomena of existing volcanoes. They found certain rocks, for the most part without stratification, and of a peculiar mineral composition, to which they gave different names, such as basalt, greenstone, porphyry, trap tuff, and amygdaloid. All these, which were recognised as belonging to one family, were called “trap” by Bergmann, from trappa, Swedish for a flight of steps—a name since adopted very generally into the nomenclature of the science; for it was observed that many rocks of this class occurred in great tabular masses of unequal extent, so as to form a succession of terraces or steps. It was also felt that some general term was indispensable, because these rocks, although very diversified in form and composition, evidently belonged to one group, distinguishable from the Plutonic as well as from the non-volcanic fossiliferous rocks.
By degrees familiarity with the products of active volcanoes convinced geologists more and more that they were identical with the trappean rocks. In every stream of modern lava there is some variation in character and composition, and even where no important difference can be recognised in the proportions of silica, alumina, lime, potash, iron, and other elementary materials, the resulting materials are often not the same, for reasons which we are as yet unable to explain. The difference also of the lavas poured out from the same mountain at two distinct periods, especially in the quantity of silica which they contain, is often so great as to give rise to rocks which are regarded as forming distinct families, although there may be every intermediate gradation between the two extremes, and although some rocks, forming a transition from the one class to the other, may often be so abundant as to demand special names. These species might be multiplied indefinitely, and I can only afford space to name a few of the principal ones, about the composition and aspect of which there is the least discordance of opinion.
Minerals most abundant in Volcanic Rocks.—The minerals which form the chief constituents of these igneous rocks are few in number. Next to quartz, which is nearly pure silica or silicic acid, the most important are those silicates commonly classed under the several heads of feldspar, mica, hornblende or augite, and olivine. In Table 28.1, in drawing up which I have received the able assistance of Mr. David Forbes, the chemical analysis of these minerals and their varieties is shown, and he has added the specific gravity of the different mineral species, the geological application of which in determining the rocks formed by these minerals will be explained in the sequel (p.504).
Analysis of Minerals most abundant in the Volcanic and Hypogene Rocks.
| THE QUARTZ GROUP | ||||
| QUARTZ | 100·0 2·6 | Silica Specific gravity | ||
| TRIDYMITE | 100·0 2·3 | Silica Specific gravity | ||
| THE FELDSPAR GROUP | ||||
| ORTHOCLASE. —— Carisbad, in granite (bulk) | 65·23 16·26 0·27 nil trace nil 14·66 1·45 nil 2·55 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Sanadine,Drachenfels in trachyte (Rammelsberg) | 65·87 18·53 nil nil 0·95 0·30 10·32 3·49 W. 0·44 2·55 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| ALBITE. —— Arendal, in granite (G. Rose) | 68·46 19·30 nil 0·28 0·68 nil nil 11·27 nil 2·61 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| OLIGOCLASE. —— Ytterby, in granite (Berzelius) | 61·55 23·80 nil nil 3·18 0·80 0·38 9·67 nil 2·65 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Teneriffe, intrachyte (Deville) | 61·55 22·03 nil nil 2·81 0·47 3·44 7·74 nil 2·59 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| LABRADORITE. —— Hitteroe, in Labrador-rock (Waage) | 51·39 29·42 2·90 nil 9·44 0·37 1·10 5·03 W. 0·71 2·72 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Iceland, involcanic (Damour) | 52·17 29·22 1·90 nil 13·11 nil nil 3·40 nil 2·71 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| ANORTHITE. —— Harzburg, in diorite (Streng) | 45·37 34·81 0·59 nil 16·52 0·83 0·40 1·45 W. 0·87 2·74 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Hecla, in volcanic(Waltershausen) | 45·14 32·10 2·03 0·78 18·32 nil 0·22 1·06 nil 2·74 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| LEUCITE. —— Vesuvius, 1811, in lava (Rammelsberg) | 56·10 23·22 nil nil nil nil 20·59 0·57 nil 2·48 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| NEPHELINE. —— Miask, in Miascite (Scheerer) | 44·30 33·25 0·82 nil 0·32 0·07 5·82 16·02 nil 2·59 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Vesuvius, involcanic (Arfvedson) | 44·11 33·73 nil nil nil nil nil 20·46 W. 0·62 2·60 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| THE MICA GROUP | ||||
| MUSCOVITE. —— Finland, in grante (Rose) | 46·36 36·80 4·53 nil nil nil 9·22 nil F. 0·67 W. 1·84 2·90 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| LEPIDOLITE. —— Cornwall, in granite (Regnault) | 52·40 26·80 nil 1·50 nil nil 9·14 nil F. 4·18 Li. 4·85 2·90 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| BIOTITE. —— Bodennais (V. Kobel> | 40·86 15·13 13·00 nil nil 22·00 8·83 nil W. 0·44 2·70 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Vesuvius, involcanic (Chodnef) | 40·91 17·71 11·02 nil 0·30 19·04 9·96 nil nil 2·75 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| PHLOGOPITE. —— New York, in metamorphic limestone(Rammelsberg) | 41·96 13·47 nil 2·67 0·34 27·12 9·37 nil F. 2·93 W. 0·60 2·81 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| MARGARITE. —— Nexos (Smith) | 30·02 49·52 1·65 nil 10·82 0·48 1·25 W. 5·55 2·99 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia =Potash =Soda Other constituents Specific gravity | ||
| RAPIDOLITE. —— Pyrenees (Delesse) | 32·10 18·50 nil 0·06 nil 36·70 nil nil W. 12·10 2·61 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| TALC. —— Zillerthal (Delesse) | 63·00 nil nil trace nil 33·60 nil nil W. 3·10 2·78 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| THE AMPHIBOLE AND PYROXENEGROUP | ||||
| TREMOLITE. —— St. Gothard (Rammelsbeg) | 58·55 nil nil nil 13·90 26·63 nil nil F.W. 0·34 2·93 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| ACTINOLITE. —— Arendal, in granite (Rammelsberg) | 56·77 0·97 nil 5·88 13·56 21·48 nil nil W. 2·20 3·02 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| HORNBLENDE. —— Faymont, in diorite (Deville) | 41·99 11·66 nil 22·22 9·55 12·59 nil 1·02 W. 1·47 3·20 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Etna, in volcanic(Waltershausen) | 40·91 13·68 nil 17·49 13·44 13·19 nil nil W. 0·85 3·01 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| URALITE. —— Ural, (Rammelsberg) | 50·75 5·65 nil 17·27 11·59 12·28 nil nil W. 1·80 3·14 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| AUGITE. —— Bohemia, in dolerite (Rammelsberg) | 51·12 3·38 0·95 8·08 23·54 12·82 nil nil nil 3·35 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Vesuvius, in lavaof 1858 (Rammelsberg) | 49·61 4·42 nil 9·08 22·83 14·22 nil nil nil 3·25 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| DIALLAGE. —— Harz, in Gabbro (Rammelsberg) | 52·00 3·10 nil 9·36 16·29 18·51 nil nil W. 1·10 3·23 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| HYPERSTHENE. —— Labrador, in Labrador-Rock (Damour) | 51·36 0·37 nil 22·59 3·09 21·31 nil nil nil 3·39 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| THE OLIVINE GROUP | ||||
| BRONZITE. —— Greenland (V. Kobell) | 58·00 1·33 11·14 nil nil 29·66 nil nil nil 3·20 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| OLIVINE. —— Carlsbad, in basalt (Rammelsberg) | 39·34 nil nil 14·85 nil 45·81 nil nil nil 3·40 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
| —— Mount Somma, involcanic (Walmstedt) | 10·08 0·18 nil 15·74 nil 44·22 nil nil nil 3·33 | Silica Alumina Sesquioxide of Iron Protoxides of Iron and Manganese Lime Magnesia Potash Soda Other constituents Specific gravity | ||
In the “Other constituents” the following signs are used: F=Fluorine, Li=Lithia, W=Loss on igniting the mineral, in most instances only Water.