An Introduction to Geology

9780217681247: An Introduction to Geology

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1905 Excerpt: ...before the geologist may be satisfied that he is dealing with an ash. The ashes which are showered upon the slopes of volcanoes situated upon the land rarely include traces of living things save occasionally the charred stumps of trees, but when the ash falls on the sea-floor it may and often does enclose the remains of the organisms which inhabited that sea, and accordingly among the rocks of the past we occasionally find ancient ashes enclosing the relics of marine creatures. The structures by which we may distinguish lavas from sediments are often very apparent. It has already been said that igneous rocks are crystalline or glassy. If glassy, their formation by cooling of molten rock is certain, and there is often little difficulty in detecting the igneous character of a crystalline rock, for the crystals which it contains differ on the whole from the commoner fragments of sediments, even when their outlines are not well marked, as is generally the case. It is rare to find a sandstone composed of the minerals which form igneous rocks, for in the processes of erosion, as already stated, some of the materials become rotted by solution, and the broken fragments become sorted. Again, the finer igneous rocks which may outwardly resemble the fine-grained sediments when the crystal-grains which form them are too small to be seen with the naked eye are usually much harder than the sediments. A study of recently-formed lavas shews us other structures which we find useful when trying to settle the origin of an ancient rock. Lavas poured out from volcanoes usually become solid at the surface when the inside is still molten. This solid surface often presents peculiar characters. Sometimes the solid crust cracks and jets of still liquid rock are squirted out, and spr...

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