MONROE AMPniTHEATER— TRACHYTES AND DOLERITES. 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 asj^ect. 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, separ- 

 ated by long intervals of erosion — 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 con- 

 siderable quantities of accessory hornblende, and also having the habit of 

 homblendic 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 waa 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, tliat Scrope included under 



