OCR Text |
Show 376 STRUCTURE, AND MODE OF ACTION hemisphere, from the Equator to the Poles. In a remote island, surrounded by exotic vegetation, beneath a sky where his accustomed stars no longer shine, the voyager often recognises with joy the argillaceous schists of his birth-place, and the rocks familiar to his eye in his native land. This absence of any dependence of geological relations on the present constitution of climates does not preclude or even diminish the salutary influence of numerous observations made in distant regions on the advance and progress of geological science, though it imparts to this progress something of a peculiar direction. Every expedition enriches natural history with new species or new genera of plants and animals : there are thus presented to us sometimes forms which connect themselves with previously long-known types, and thus permit us to trace and contemplate in its perfection the really regular though apparently broken or interrupted network of organic forms : at other times, shapes which appear isolated-either surviving remnants of extinct genera or orders, or otherwise members of still undiscovered groups, stimulating afresh the spirit of research and expectation. The examination of the solid crust of the globe does not, indeed, unfold to us such diversity and variety; it presents to us, on the contrary, an agreement in the constituent particles, in the superposition of the different kinds of masses, and in their regular recurrence, which excites the admiration of the geologist. In the chain of the Andes, as in the mountains of Middle Europe, one formation appears, as it were, to summon to itself another. Rocks of the same name exhibit the same outlines; basalt and dolerite form twin mountains; dolomite, sandstone, and porphyry, abrupt precipices; and vitreous feldspathic trachyte, high, dome-like elevations. In the most distant zones, large crystals separate themselves in a similar manner from the compact texture of the primitive mass, as if by an internal development, form groups in association, and appear associated in layers, often announcing the vicinity of new, independent formations. Thus in any single system of mountains of considerable extent we see the whole inorganic substances of which the crust of the earth is composed represented, as it were, with more or less distinctness; yet, in order to become completely acquainted with the important phenomena of the com- |