| OCR Text |
Show 360 TRAP ROCKS. [Ch. XXV. rocks composed chiefly of augite, hornblend~, and felspar, which are now admitted by all to have been once in a state of fusion, they were divided in opinion whether they were of igneous or of aqueous origin. We have shown in our sketch of the history of geology in the first volume, how much the polemical controversies on this subject retarded the advancement of the science, and how slowly the analogy of the rocks in question to the products of burning volcanos ~as recognized. Most of the igneous rocks first investigated ·in Germany, France, and Scotland, were associated with marine strata, and in some places they occurred in tabular masses or platforms at different heights, so as to form. on the sides of some hills a succession of terraces or steps, from which circumstance they were called 'trap' by Bergmap. (from trappa, Swedish for a staircase), a name afterwards adopted very generally into the nomenclature of the science. When these trappean rocks were compared with lavas produced in the atmosphere, they were found to be in general less porous and more compact ; but in this instance the terms of comparison were imperfect, for a set of rocks, formed almost entirely under water, was contrasted with another which had cooled in the open air. Yet the ancient volcanos of Central France were classed, in reference probably to their antiquity, with the trap rocks, although they afford perfect counterparts to existing volcanos, and were evidently formed in the open air. Mont Dor and the Plomb du Can tal, indeed, may differ in many respects from Vesuvius and Etna in the mineral constitution and structure o( their lavas ; but it is that kind of difference which we must expect to discover when we compare the products of any two active volcanos, such as Teneriffe and Hecla, or Hecla and Cotopaxi. The amygdaloidal structure in many of the trap formations proves that they were originally cellular and porous like lava, but the cells have been subsequently filled up with silex, car· bonate of lime, zeolite, and other ingredients which form the nodules. Dr. Macculloch, after examining with great attention the Ch. XXV.] RELATIONS OF GRANITE AND TRAP. 361 igneous rocks of Scotland, observes ' that it is a mere dispute about terms to refuse to the ancient eruptions of trap the name of submarine volcanos, for they are such in every essential point, although they no longer eject fire and smoke*.' The same author a1so considers it not improbable that some of the volcanic rocks of the same country may have been poured out in the open airf. The recent examination of the igneous rocks of Sicily, cspeciall y those of the Val di N oto, l1as proved that all the more ordinary varieties of European trap have been produced under the waters of the sea in the Newer Pliocene period, that is to say, since the Mediterranean has been inhabited by a great proportion of the existing species of testacca. We are, therefore, entitled to feel the utmost confidence, that if we could obtain access to the existing bed of the ocean, and explore the igneous rocks poured out within the last 5000 years beneath the pressure of a sea of considerable depth, we should behold formations of modern date scarcely distinguishable from tl1e most ancient trap rocks of our island. We cannot, however, expect the identity to be perfect, for time is ever working some alteration in the composition of these mineral masses, as, for example, by converting porous lava into amygdaloids. Passage from t·rap into g·ranite.-lf a division be attempted between the trappean and volcanic rocks, it must be made between different parts of the same volcano,-nay even the same rock, which would be called 'trap,' where it fills a fissure and has assumed a solid crystalline form on slow cooling, must be termed volcanic, or lava, where it issues on the flanks of the mountain. Some geologists may perhaps be of opinion that melted matter, which has been poured out in the open air, may be conveniently called volcanic, while that which appears to have cooled at the bottom of the sea, or under pressure, but at no great depth from the surface, may be termed 'trap;' but we believe that such distinctions will lead only to confusion, and that we must consider trap and volcanic as synonymous. On the other hand, the difficulty of discrimi- • Syst. of Geol., vol, ii, p. 114. t Ibid· |
