AN OCCURRENCE OF GREENSTONE SCHISTS IN THE 

 SAN JUAN MOUNTAINS, COLORADO. ' 



In the course of a recent examination of the Needle Mountains 

 quadrangle by the United States Geological Survey, a series of meta- 

 morphic rocks was encountered that differs in many respects from 

 those occurring near by in the Animas Canyon, which have been con- 

 sidered to be of Archean age. 



The region is near the southwestern limits of the San Juan Moun- 

 tains of southern Colorado, which are made up largely of Tertiary 

 volcanic rocks. In that portion which is known locally as the Needle 

 Mountains, and which lies in parts of San Juan and La Plata counties, 

 the younger lava flows and breccias are absent, and ancient crystal- 

 line or metamorphic rocks have been exposed by the dissection of a 

 dome-like uplift, in which all of the sedimentary formations, as late 

 at least as the last of the conformable Cretaceous beds, have been 

 involved. These rocks, which are all of pre-Cambrian age, are 

 granites, schists, and quartzites; the ones to be described, which may 

 be referred to conveniently as greenstones, occur at the southern 

 side of the uplift. 



During a hurried visit to the region in 1901, one of the members 

 of the party, who was familiar with the Marquette and Menominee 

 greenstones, called attention to the similarity of these rocks to those of 

 the Lake Superior region. In the next field season a more detailed 

 study was made of the complex, and in the laboratory specimens of 

 the Needle Mountains rocks were compared with those collected 

 by the late G. H. Williams in the Marquette and Menominee locali- 

 ties, as well as with specimens described by Cross from near Salida, 

 Colorado, in the Arkansas Valley. 



Occurrence of the greenstones. — The greenstones are found for a 

 little over seven miles in a north-and-south direction on both sides of 

 Vallecito Creek, midway between its head and the point where it 

 joins Pine River. From east to west the area occupied by the green- 

 stones is not more than two and a half miles wide at most. To the 



I Published by permission of the Director of the U. S. Geological Survey. . 



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