THE HINSDALE (iNEISS AXD LIMESTONE. 25 



nodules are placed in lamination ])lanes about 30'""' apart, tlic interspace, 

 except for rare tliin films of the same, being made up of a dead-white mix- 

 ture of much feldspar and little quartz, mostly fine grained, but with here 

 and tliere large curved cleavage faces of orthoclase exj)Osed. It contains 

 ]iyrite in small pentagonal dodecahedrons and submicroscopic zircons of 

 dark clove-ljrown color. Under the microscope tlu^ rock is nuicli dusted 

 with minute inclusions which give it an opaque white a])pearance. The 

 quartz contains a few short, straight, black microlites, unlike the long 

 rutile needles of the granites. The trains of cavities are vevv abundant, 

 and often run through several grains of quartz, suggesting crushing. The 

 orthoclase shows all stages of decomposition into epidote. At the begin- 

 ning the epidote gathers in small crystals in the two cleavage planes. The 

 microcline is filled with the same short, black microlites as the quartz, and 

 shows most beautiful microcline structure. The only place where a rock 

 of this type appears in the old Ham})shire County area is in the coarse mass 

 which adjoins the Coles Brook limestone on the west. It contains, as does 

 the pre-Cambrian gneiss of the Tyringhani Valley farther south, a white 

 orthoclase in large cleavage plates, which exhibits a rich l)lue opalescence. 



THE HINSDALE LIMESTONE, HINSDALE. 



Fifty rods west of Hinsdale station the limestone occurs Avith an 

 exposed thickness of 25 feet (the top not seen) and dips 30° E. Eight 

 hundi-ed and thirty feet farther west, at a stone mill, a gray epidotic gneiss 

 occurs, with strike 30° S. and dip 65° E. 



The limestone is a white to pijak, rather coarse (grains 3-5™"'), highly 

 crystalline rockj with a certain translucency in the grains which distin- 

 guishes it immediately from all the other limestones of western Massachu- 

 setts and allies it to the limestones of the Adirondacks. It carries coccolite, 

 phlogopite, biotite, actinolite, chondrodite, pyrite, and magnetite. Generally 

 the coccolite or the chondrodite, or both, are so al)undantly and CA-enly 

 scattered through the mass that it deserves the name coccolitic limestone or 

 chondi'oditic limestone, and tlie accessory minerals are so arranged as to 

 give the mass a distinct foliation, esjjccially when the chondrodite and 

 biotite predominate. 



