484 Russell — Relations of the Great 



spring of 1922. The present article is an abridgment of 

 an essay submitted to Yale University for the degree of 

 master of science. The writer is greatly indebted to 

 Prof. C. E. Longwell and Prof. Adolph Knopf, of Yale 

 University, for advice and criticism in preparing the 

 report. 



General Description of the Formations. 



1. The crystallines and altered crystallines. — The 

 rocks to the east and sonth-east of the great fault consist 

 of granites, gneisses, and schists. At a distance from the 

 fault the granites near Lighthouse Point show little or no 

 evidence of shearing or gneissic banding. Near the fault 

 plane they frequently present a streaky appearance, due 

 to the crushing and drawing out of the minerals by shear- 

 ing. Under the microscope the normal granites may be 

 seen to be subject to strong cataclastic deformation, but 

 the sheared rocks are much more intensely deformed, 

 and some of them have been reduced to mylonites. The 

 minerals have been crushed and granulated until they are 

 mere lines, and sericite has been developed along shear 

 planes and zones of microbrecciation. These sheared 

 rocks and mylonites are found at intervals along the 

 course of the fault. They were not observed at a 

 distance from it, and the amount of shearing increases 

 irregularly towards the fault plane. The sheared rocks 

 are often cut by quartz veins which are not sheared. 

 North of Branford, where these rocks are best exposed, 

 the shear planes are nearly horizontal, or dip slightly 

 towards the fault. 



2. The quartz lode. — One of the remarkable features 

 of the region is the presence along the eastern edge of the 

 fault plane of a quartz lode of considerable extent and 

 size, which affords indications of the age of the fault, and 

 the dip of the fault plane. In general this lode consists 

 of white, bluish, or greenish quartz, cut by a network of 

 veins of white, coarsely granular quartz. In addition 

 there are occasionally cubical crystals of pyrite, and 

 rarely nuorite. A composite sample of the lode con- 

 tained $0.31 of gold to the ton. On its eastern border 

 this vein in many places passes into the crystallines by a 

 gradual transition from the quartz of the lode to altered 

 crystallines partly replaced by quartz, and finally to 



