126 Q. B. Grinnell and K S. Dana— Tertiary Lake Basin. 



If we assume that the pyroxene of this rock has the same 

 composition as that of No. 1, we calculate, from the magnesia 

 that it contains, 44 per cent of this ingredient; then, deducting 

 tliree per cent of titanic iron, we have left a remainder of 63 

 per cent, which has very exactly the ratio and composition of 

 oligoclase. This mineral constitution appears to be justified by 

 the microscopic examination, since no free quartz or other 

 mineral can be detected. If we restrict the use of the term 

 melapbyre, as it is done in some recent works on lithology, to 

 of oligoclase and pvroxene, ' ' ' 

 ^ ^.ismelar " 



the rock here analyzed is melapbyre in composition as well as 

 appearaTice ; and being a metamorphic rock, it is metamelaphyre. 

 We thus have representatives of the larger part of the pyrox- 

 enic igneous rocks, in positions whieb show conclusively that 

 tbey are of metamorphic origin. The fact that metamorphic 

 action can produce rocks exactly like the igneous in external 

 aspect and cbemical constituents is of great interest in the 

 study of rocks. 



Several Lake Basins of Tertiary age bave already been 

 discovered in tbe Rocky Mountaiii region, and the more im- 

 portant of tbem have been carefully explored. Those of 

 Eocene age have only been known since 1870, but the Miocene 

 deposits of the White River have long been noted for their 

 wonderful scenery, as well as for the number and variety of the 

 mammalian remains found in them. Another Miocene basin 

 is known in Oregon, and both tbe lake beds of this period are 

 overlaid by deposits of Pliocene age.* 



During the explorations carried on last summer under tbe 

 direction of Col Wm. Ludlow, Corps of Engineers, a series of 

 Tertiary deposits were identified by the writers near Camp 

 Baker, 'Montana. These deposits indicate the existence in this 

 region of a Miocene lake basin, which was succeeded by an- 

 other lake basin in Pliocene time. As tbese basins are quite 

 distinct from those heretofore known, it is considered important 

 to put the fact of their discovery on record. 



Camp Baker is situated on Deep Creek, a stream which flows 

 into the Missouri River above Sun River. It lies about fifty 

 miles nearly due east of Helena. It is surrounded on all sides 

 by mountains, of which tbe Big Belt Range, lying immediately 

 to the south or southwest, is the highest and most conspicuous. 

 * This Journal, III, vol. ix, p. 49, Jan., 1875. 



