and other Rocks from Montana. 287 



basic composition. Dr. C. A. White has had the kindness to 

 furnish me with information regarding the occurrence in the 

 field of the rocks and all the statements in the following 

 pages in reference to this are from his unpublished notes. 



The Moccasin Mountains form an isolated group about 

 seventy miles southeast from Fort Benton on the eastern side 

 of Judith river. They consist, according to Dr. White, of a 

 central core of eruptive masses, probably laccolitic in charac- 

 ter, surrounded by a ring of upturned sediments ranging from 

 the Cretaceous to the Carboniferous. The rocks from the 

 central core appear to belong only to one type. They are 

 light colored, gray or yellowish porphyritic rocks with large 

 phenocrysts of sanidine and soda-lime-feldspar, and smaller 

 ones of brown hornblende ; a greenish augite is occasionally 

 seen. The groundmass is always holocrystalline of more or 

 less fine grain and consisting of unstriated feldspar and quartz 

 which sometimes are intergrown in such manner that each 

 quartz grain contains numerous smaller feldspar grains of 

 irregular optical orientation.* 



These intrusive rocks from the Moccasin Mountains appear 

 almost identical with the laccolitic masses in the Carboniferous 

 of the Little Belt Mountains and the dikes in the Cretaceous 

 east of Cadottes pass which I have described in previous 

 papers under the name of dacites and diorites.f 



Again, very similar rocks have been described in detail by 

 Mr. Whitman Cross from Leadville and other localities in Colo- 

 rado, and by Mr. Iddings from the Yellowstone Park region. 

 It is apparent that this type of porphyritic intrusive rock is of 

 wide-spread occurrence in the Rocky Mountains ; the name of 

 porphyrite or quartz-porphyrite^: is now usually applied to 

 them. 



The Bear Paw Mountains are situated sixty miles northeast 

 of Fort Benton and rise about 2500 feet above the surround- 

 ing plains. According to Dr. White, they are largely made 

 up of igneous rocks intruded in Cretaceous strata. The speci- 

 mens which are taken in the broad valley of Eagle creek 

 at the south base of the mountains, mostly from dikes or dike- 

 like masses, show a very different type from the one just 

 described. 



Prevailing is a dark fine-grained porphyritic rock with 

 phenocrysts of greenish idiomorphic augite up to three milli- 



* The name of micro-poikilitic has recently been suggested by Mr. Iddings for 

 this structure. 



f 10th Census, vol. xv, pp. 720 and 731. Eruptive rocks from Montana; Proc. 

 Cal. Acad. Sci., series II, vol. iii, p. 39. 



% The definition of porphyrite as applied to this type of rocks is given by Mr. 

 Iddings in his paper on " The Eruptive Rocks of Electric Peak and Sepulchre 

 Mountain." 12th Ann. Rep. U. S. Geol. Survey, p. 582. 



