notes ok some eruptive rocks from gallatik, 

 jeeeerso:n^, anb madisois^ cou^^ties, Montana. 



By George P. Merrill, 



Curator of the Departvunt of Geology. 



The rocks described below were collected by Dr. A. C. Peale and 

 the writer, mainly during the seasons of 1887, 1888, and 1889. The 

 writer's own observations were limited to two brief seasons in 1887 and 

 1889. The region covered is quite extensive, comprising upward of 

 200 square miles as shown on the Three Forks sheet of the U. S. Geo- 

 logical Survey, and much of it difficult of access. In many instances 

 doubtful points regarding the occurrence and association of certain 

 masses could have been decided only by a second visit to the locality, 

 after the first series of specimens collected had been submitted to study 

 in the laboratory and we were in a condition to avail ourselves of the 

 knowledge thus gained. Unfortunately, this we have been unable to do, 

 and while in many instances we are led to infer that somewhat variable 

 rock tyi^es are but widely separated facies of the same magma, we 

 have no absolute proof of the same. Eruptive rocks of a wide geolog- 

 ical range and of widely varying character are abundant throughout 

 tlie region. Here I shall attempt to describe oidy those of greatest 

 petrological interest. A few of them have been noted in a preliminary 

 Avay in Bulletin No. 110, U. S. Geological Survey, 1894. 



In describing the rocks they will be taken up as found along the 

 main routes traveled, in the following order: (1) From the foothills 

 north of Gallatin Peak, along the valley of the Gallatin and East 

 Gallatin to the Horse Shoe Bend of the Missouri River; (2) from 

 Three Forks southward, up the Madison Valley and into the foothills 

 on either side as far as the Wedge and westward to Virginia City; 

 (3) from the same jioint, southwestward, up the Jefferson River as far 

 as South Boulder Creek. The numbers referring to specimens are those 

 given in the catalogues of the U. S. National Museum. 



Proceediugs of the U. S. National Museum, Vol. XVII— No. 1031. 



687 



