24 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



gTiiuitoid structure, which disappears insensibly as the beds recede from 

 this center. 



THE TOLLAND AREA. 



A large outcrop of Algonkian limestone occurs on the roadside in 

 Riverton, Connecticut, near the southwest corner of the Grranville quad- 

 rangle,^ associated with a coarse fibrolitic two-mica-gneiss, which is charac- 

 teristically Algonkian in the Sandisfield quadrangle, next west of this, 

 where it is a variant of the blue-quartz gneiss. This fibrolite-gneiss enters 

 the Granville quadrangle at its southwest corner and runs north by east, in 

 a tapering syucline, to a point north of Black Pond in Tolland. 



The rocks often resemble coarse mica-schists, and are scarcely distin- 

 guishable from the coarse schists and schistose gneisses on the horizon of 

 the Hoosac and Rowe schists, which lie next east of the Becket gneiss, 

 except that they contain fibrolite and lie beneath the Cambrian gneisses, 

 and in the next quadi'angle west can be traced into undoubted connection 

 with the blue-quartz gneisses and the chondroditic limestones. 



East of the middle of the town of Tolland, at 0. E. Slocum's,^ is a 

 great quantity of large bowlders of a peculiar coarse hornblendic gneiss, 

 often brecciated, with black hornblende, colorless quartz, and orthoclase. 

 Some masses are medium-grained, some coarse, with hornblendes 4 to 

 5 inches long and 1 inch square at base. This rock is mentioned by 

 President Hitchcock, but I could not find it in jjlace. It jjrobably was 

 derived from the Algonkian anticline to the west. 



PETROGRAPIIICAL r)ESCRIPTIO:N". 



LOWER OR HINSDALE GNEISS, HINSDALE STATION. 



The coarse gneiss just above ^ the limestone is granitoid in texture 

 and contains in abundance a fi-esh black biotite in large scales, which in its 

 uj)per layers are aggregated into concretionary masses, flattened-out lentic- 

 ular nodules made up wholly of fine scales of biotite and epidote. These 



' The four-cornered division of the earth's surface represented on one of the sheets of the Topo- 

 graphic Atlas of the United States is called a quadrangle. 



- The manuscript of this work was mostly completed before the atlas sheets of the United States 

 Geological Survey were issued, and the citation of names refer to those upon the county atlases of 

 F. W. Beers. 



3 Stratigraphieally below, as the rooks are overturned. 



