244 G. C. Martin — Dunite in Western Massachusetts. 



Art. XX V. — An Occurrence of Dunite in Western Massa- 

 chusetts ;* by G. C. Martin. 



Introduction. — The occurrence of a mass of dunite in the 

 Green Mountain region of Massachusetts is of interest and 

 importance because it is a feature of the geology of that region 

 which lias apparently been hitherto overlooked, because there 

 are on record only two other true d unites in North America, 

 and because its mode of occurrence is such as to leave little 

 doubt of its eruptive origin. 



Location and Topography. — The rock occurs on the moun- 

 tain south of Cheshire. This mountain is a spur of the Hoosac 

 Range and although it is in the territory discussed by Wolff in 

 his " Geology of Hoosac Mountain "f it is topographically dis- 

 tinct from the main mass of that range. This is shown on the 

 contour maps, but is much more distinctly shown in the view 

 from some of the neighboring peaks. The exact location of 

 the dunite area may be found upon the Greylock sheet of the 

 topographic map, or upon the geological map (Plate I) of 

 Monograph XXIII U.S. Geol. Survey, by tracing the meridian 

 73° 10' for 2J miles south from the village of Cheshire to its 

 intersection with an unnamed stream. The area in question lies 

 chiefly between this stream and Whipple Brook, the next to 

 the southward. The main dunite mass occupies an area irregu- 

 larly oval in outline and about 1000 by 2000 feet in extent. It is 

 situated upon a flattened shoulder on the mountain-side, and is 

 separated from the surrouuding gneisses by brooks on the 

 north and south, and by swamps on the east and west. Within 

 the area are a number of rocky ridges 10 to 50 feet high, sepa- 

 rated by swamps. About 400 feet northeast of the main mass 

 is a smaller one of uncertain extent. (Loc. 351 as shown on 

 the accompanying map.) Another exposure of doubtful rela- 

 tion to the main mass is east of the north end of it at Loc. 

 114 c. • while at numerous localities east and southeast of this, 

 and northwest of the main mass at Loc. 304 the proximity of 

 dunite is indicated by abundant fragments, some of which may 

 be practically in place. In the banks of the Whipple Brook 

 and at many places to the south, possible outcrops are to be 

 seen, but since they may be merely glacial bowlders they are 

 not mapped. Between all these and the main mass outcrops 

 of gneiss intervene and no surface connection is visible. 



Description of Rocks. — The rocks of this area vary much 

 in appearance ; but as will be seen, the change from one form 



* The author is indebted to Dr. A. C. Gill for advice and assistance in the 

 preparation of this paper, especially in the chemical analysis, which is largely Dr. 

 Gill's own work. 



f Monograph XXIII, U. S. G. S., Part II. 



