Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER IV. VOLCANIC ROCKS. No clear distinction can be drawn between plutonic rocks and coarsely crystalline forms of volcanic rocks. Both are extruded in some instances from deep-seated parts of the earth. In consequence of the rigid condition of the globe it is impossible that those rocks came to the surface from an unconsolidated interior by ascending fissures. Many writers have assumed the existence of molten areas or lakes in the interior of the crust, as a source for lava streams, which sometimes flow on the surface for a hundred miles. Others, again, assume that the longitudinal fissures, along which volcanic cones have been built, penetrate down to different layers of the earth, each distinguished by having the mineral character of the different kinds of volcanic rocks. Such a fissure allows the atmosphere to penetrate downwards, and removes from the heated rock the pressure which had kept it solid. The rock then liquefies and ascends the fissure like fluid in a pump, until it comes in contact with water derived from the earth's surface, and so generates steam, which forms the explosive outbursts. The steam ascends miles high into the air, carrying up the rock in the form of dust. The dust from the volcano Krakatoa, in the Strait of Sunda, ejected in 1884, remained suspended for more than a year. On this hypothesis the difference between plutonic rocks and volcanic rocks is in the circumstance that the plutonic rocks consolidate deep in the earth, while the volcanic rocks consolidate under the pressure of the atmosphere, or near to the surface. The principal types of volcanic rocks are named Rhyolites, Trachytes, Andesites and Basalts. The basalt has been supposed to be the last formed; and to have come from a greater depth than the others, being commonly t...