658 



ERUPTIVE BOCKS FROM MONTANA— MERRILL. 



VOL. XVII. 



as given by him. HoDibJendite includes the ch^sely related hornblende 

 augites forms and hypersthenite the pure hypersthene rock. The com 

 pound namehornblende-hyperstlienite, while sufiQciently descriptive, is 

 too cumbersome, but it seems scarcely advisable to coin a new name 

 until* rocks of this type shall be shown to have a wider geographic 

 distribution. 



Below is given the results of a bulk analysis as made by Mr. Eakins 

 (i). In II is given the composition of a bronzite diallage rock (Web- 

 sterite) from near Webster, N. C, as described by G. H. Williams.* 



Between South Meadow and Moore Creek was found a second incon- 

 spicuous outcrop'of what is evidently a varietal form of the same rock 

 (No. 02401, U.S.N. 31.). As in the last case, the outcrop is in the gneiss, 

 nearly circular in outline, and of very limited area, not over 100 feet 

 in greatest diameter. The actual contact between the eruptive and 

 the gneiss was obscured by a zone, some three or four feet wide, of decom- 

 posed material, and here again there may be some reason to doubt the 

 eruptive natureof the rock. To consider it as an. eruptive is certainly 

 the easiest way out of the difficulty, since it is more difficult to explain 

 how mineral aggregates of this nature could segregate out of gneissic 

 rocks of entirely ditferent mineralogical composit on than it is to 

 account for the coarse and uniform crystallization in eruptive masses 

 of so snuxll size. 



The rock in the hand specimen is dense, dark greenish in color with 

 a serpintinous look and flecked with abundant cleavage plates of dark- 

 green hornblende. On the immediate surface the rock weathers to a 

 rusty red and shows not infrequently small rounded garnet-like pro- 

 tuberances, which closer examination shows to be large hypersthenes 

 left projecting, owing to their superior durability. 



In the thin section the rock shows plates of faintly greenish, to almost 

 colorless hornblendes, interspersed with short, stout hypersthenes and 

 occasionally olivines, and very abnnda'it, comparatively large, irregulai" 

 deep green pleonasts with which is nearly always associated a magnetic 

 iron ore which gives a chromium reaction when tested in the borax 

 bead. 



The rock is very tresh, although a slight serpentinization has begun 



Am. Geologist, J«iy, 1890, p. 44. 



