680 L. V. FIRS SON 



found to consist chiefly of silica, alumina, soda and water. Thus, 

 as Rosenbusch pointed out, it had certain analogies with an 

 eleolite-syenite magma from which the partial magma forming 

 the dikes is supposed to be formed by differentiation of the 

 lime, iron and magnesia. On account of the water it is called a 

 pitchstone glass. 



Within the past few years the attention of the author has 

 also been directed to this group of rocks by their occurrence in 

 parts of Montana now being studied in conjunction with Mr. W. 

 H. Weed, under the auspices of the United States Geological 

 Survey. When they first appeared the characterization of them 

 as consisting of ferro-magnesian minerals in a glass base, given 

 by previous authors, was accepted, and one of them so described 

 (in the report on the geology of the Castle Mountain mining 

 district, now passing through the press) . 



As, however, the number of examples increased and the 

 rocks were studied in connection with their geologic mode of 

 occurrence, it became a source of perplexity as to why such 

 basic magmas, solidifying under the conditions which the gen- 

 eral geology of the region evinces, should have formed so much 

 glass. 



For it must be said that a priori one would scarcely expect 

 to find such basic magmas as form the monchiquites, producing 

 glass when solidifying at such depths as they have in the cases 

 which have come under our own observation, and in the instances 

 which have been described by others. This anomaly is all the 

 more marked when the acid dikes and intrusive sheets which so 

 generally accompany them, and which must have been formed 

 under similar conditions, are taken into account. As is well 

 known the acid magmas crystallize with more difficulty than 

 the basic ones ; rhyolitic glasses are well known and are com- 

 mon, while tachylites are rare ; at the same depths and under 

 the same conditions which cause the very acid magmas to form 

 extremely fine-grained dense or porphyritic rocks the basic 

 magmas crystallize into moderate or even coarse-grained evenly 

 granular ones. If the monchiquites contained so much glass 



