Page:A Treatise on Geology, volume 2.djvu/103

 CHAP. VII. Von Buch speaks of the transition of "gabbro," or diallage rock, to granite, in the island of Kielvig.

No author has given more attention to the transitions which obtain between the various pyrogenous rocks, nor with greater success, than the late Dr. MacCulloch, to whom, indeed, modern geologists owe a large debt, for the clear and masterly conceptions he has published on this subject. He tells us, concerning the granites of Aberdeen shire, which are generally composed of quartz, felspar, and mica, that in this compound hornblende is occasionally substituted for mica; that the quartz sometimes fails; that this hornblendic mass becomes fine grained, and passes to greenstone, basalt, and an earthy trap-like claystone.

Von Dechen, in the German translation of De la Beche's manual, expresses very clearly the state of opinion among geological observers, as to the gradation in character from one to another, of all the igneous rocks. Thus granite, by replacement of its mica with hornblende, changes to sienite; by containing detached felspar crystals, it becomes porphyritic; and when reduced to very fine grains, we can entirely corroborate Von Dechen in saying that it is indistinguishable from felspar porphyry. A more earthy basis gives us clay porphyry; a concentric internal arrangement makes globular porphyry (kugel porphyr).

Trachyte and porphyritic trachyte are a parallel series to granite and porphyritic granite; in an earthy state they constitute domite.

Sienite and felspar porphyry pass by variation of mineral ingredients to the vague group of greenstones or traps, in which hornblende or augite forms a prominent part of the mass. Of these, diorite (diabase or greenstone) is related to sienite (the gradations being called greenstone sienite, and sienitic greenstone, &c.). The total absence of felspar turns such greenstones into hornblende rocks; diorites with extremely fine grains are called aphanite, and these cannot often be separated from the more quartzose rocks, usually called hornstone (by