Geology for beginners
William Whitehead Watts
Paperback
(RareBooksClub.com, March 6, 2012)
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. 1903 Excerpt: ...surface-products may never have been formed (Fig. 122). Gradation towards Volcanic Rocks.--But if these rocks are really the roots of volcanoes, we ought to find eveiy F1g. I2z.--Dyke of gra-1ite, Brazil Wood, Leicestershire. G = granite; S = metamorphosed shales. transition from them to the volcanic rocks. This is, in fact, the case, for granites themselves vary in grain from coarse to fine, and from porphyritic to very compact types. They give off dykes and the more irregular protrusions called veins, which, as they are traced nearer the surface and away from the main granite-mass, become finer in grain and sometimes take on a stony texture (Fig. 123). At times their edges and ends are even glassy or vesicular, and thus the transition from granite through dyke-rocks to volcanic rocks is quite complete and gradual. A mass of intrusive rock may shrink in cooling, and parts of the uncooled magma may be intruded into that which has been already solidified. Thus veins will be formed which may have a slightly different composition from that of the part first soldified. Granite in chemical and mineralogical composit1on--N felspar, quartz, and sometimes hornblende--corresponds with the lava-rock rhyolite; but there are also rocks corresponding to the other volcanic products. The whole group is called Plutonic,1 from the circumstance of its deep-seated origin. F1g. 123.--Veins of granite (G) piercing slates (S) of Tremadoc age, and enclosing patches of them; Tan-y-Grisiau, Carnarvonshire. (From a photograph by Mr. Griffith Williams: copyright.) Thus there are acid, intermediate, and basic plutonic rocks which correspond, species for species, with the volcanic rocks, and the table on page 168 may be extended as follows:--Class1f1cat1on Of Igneous Rocks Dykes, Sills...