GREENSTONE SCHISTS IN SAN JUAN MOUNTAINS 507 



Age of the greenstones.— The oldest rocks in contact with the 

 Irving greenstones are the conglomerates and quartzites of the 

 Algonkian to the north and east. The actual relations of the two 

 formations have been obscured by faulting or infolding, but the 

 greater age of the Irving is shown by the quantities of schistose and 

 massive greenstone pebbles in the lower portions of the Algonkian 

 conglomerate. The character of these pebbles also supplies infor- 

 mation in regard to the age of the mashing of the Irving greenstones, 

 the greater part of which evidently took place before the deposition 

 of the conglomerate. Later movements occurred after Algonkian 

 time and resulted in the further fracturing and mashing of the green- 

 stones, as well as the lower portions of the conglomerate. 



As to the lower age limits, little can be said with certainty, except 

 that the rocks have been less affected by dynamic metamorphism 

 than any that are known to occur in the neighboring Archean areas. 

 They have been regarded as of early Algonkian age, separated from 

 the younger Algonkian sediments by an erosion interval of unknown 

 extent. That the Irving, as now exposed, represents but a small 

 part of a much greater series of rocks which formerly existed seems 

 certain. Evidence of this is to be found in the great thickness of 

 Algonkian conglomerates, for, although greenstone pebbles are con- 

 spicuous in many places, quartzite debris is more abundant, and 

 lenses or beds of magnetite or jasper bowlders are often seen high up 

 in the section, all of which indicates the destruction of an earlier 

 terrane, the only traces of which that may now be recognized being 

 the comparatively rare beds of quartzite and magnetite associated 

 with the Irving greenstones. These same remnants are believed to 

 be inclusions in the diabase and gabbro which intruded sediments 

 that have since been worn down to supply the materials for the 

 younger series of quartzites. 



Comparison with rocks of other localities. — The collection of rocks 

 from the Menominee and Marquette regions made by the late G. H. 

 Williams, and described by him in a bulletin of the United States 

 Geological Survey,' has been examined in connection with the pres- 

 ent study, and a certain similarity between the two series of rocks 



^ G. H. Williams, "The Greenstone Schist Areas of the Menominee and Mar- 

 quette Regions of Michigan," Bulletin No. 62 (1890), U. S. Geological Survey. 



