OCR Text |
Show MONEOE AMPHITHEATEEâ€"TEACIIYTES AND DOLEEITES. 231 cleavage being sometimes parallel with the bedding, sometimes making a large angle with it, like slate. Hornblende, augite, and black mica, in very small crystals, are sparingly disseminated through it. Associated with these are masses of doleritic lava. I use this designation to indicate a rock more basic than andesite, but less so than basalt; and though more nearly approaching the latter, is distinguished from it both in mode of occurrence and in aspect. It is associated with the middle eruptions and I believe never with the later. Its feldspars are tri clinic (Labradorite), frequently in large crystals, which have a conspicuous glassy luster, resembling sani-din. It never contains olivin. Usually it is blackish and nearly as dark as basalt, but in some cases it is red, even in compact specimens. We have, then, in this great amphitheater more than 4,000 feet of volcanic rocks, belonging to at least two periods, and possibly more, separated by long intervals of erosionâ€"the oldest going back into the latter part of the Eocene, the younger belonging to I know not what period exactly, but from general considerations, am disposed to regard them as Miocene or early Pliocene, covering a long period in their totality, which may extend throughout the entire range of Miocene and Pliocene time. At the base of the series we find large bodies of rock, consisting of plagioclase, with considerable quantities of accessory hornblende, and also having the habit of hornblendic propylite and hornblendic andesite. These were much eroded after their eruption and before the extravasation of the later coulees. They are succeeded by heavy masses of rather fine-grained augitic andesite in great sheets, reaching a thickness of .300 and even 400 feet, and are followed by equally heavy masses of trachyte, sometimes augitic, sometimes with no great or notable amount of any accessory mineral. With these last doleritic eruptions intercalate Scrope, in his work on the " Volcanoes of Central France," repeatedly mentions the occurrence of " basalts" intercalating with the trachytic masses of Mont Dore and the Cantal. He wa» particular to call attention to the fact that in that region no confirmation was found of the view which had been entertained by some geologists that the basalts were erupted at a later period than the trachytes, and notes many instances where " basalt" was overlaid by trachyte. It is clear, however, that. Scrope included under |