C. Palache — Occurrence of Olivine. 491 



Art. XLYIII. — Occurrence of Olivine in the Serpentine of 

 Chester and Midcllefield, Mass. ; by Charles Palache. 



Some years since, while examining the private mineral collec- 

 tion of Mr. E. L. Cowles of Chester, the writer's attention was 

 drawn to certain specimens which were identified as olivine, 

 a mineral which had long been sought for in the region but 

 without success. Mr. Cowles was so kind as to supply speci- 

 mens for study and, on a later excursion to Chester, conducted 

 the writer and several students to the locality where he had 

 found the olivine, giving us opportunity to collect abundant 

 material and to see the nature of the occurrence. At the same 

 time specimens of the serpentine, which occurs in a large mass 

 at the locality, were obtained and olivine was found in the rock 

 in subsequent study of thin sections. 



As considerable interest attaches to these occurrences of 

 olivine, publication of the observed facts seems desirable. 



Both occurrences of olivine are in a lense-shaped mass of ser- 

 pentine, about a mile and a half long and nearly a half mile 

 wide, that extends from the town of Middlefield into the town 

 of Chester. According to Professor Emerson* this serpentine 

 contains chromite locally, and also supplied the specimens of 

 serpentine pseudomorphs after olivine known as "hampshirite," 

 to which reference will again be made in these pages. But not- 

 withstanding these suggestions of the derivation of the serpen- 

 tine from a peridotite, Emerson was unable to definitely deter- 

 mine olivine in any of his slides, which were made from the 

 western half of the bed, and came to the conclusion that the 

 serpentine was in large part at least derived from the associated 

 amphibolite and not from olivine. Professor Emerson found 

 much olivine in the continuation of this bed to the south and 

 much coarse enstatite rock. 



In thin sections of a massive dark green serpentine collected 

 near the eastern boundary of this serpentine area where the 

 Chester-Middlefield road crosses it, olivine was found in abun- 

 dance, in complete anhedra and as centers of a network of platy 

 serpentine developed in characteristic fashion by the alteration 

 of the olivine. Much of the serpentine in the slides bore marks 

 of the same derivation ; other smaller areas had a different charac- 

 ter, suggesting rather the alteration of a pyroxene, but no fresh 

 pyroxene was seen. Grains of magnetite are sparsely present. 

 In every detail the specimen is a typical peridotite and seems 

 conclusive evidence to the writer of the igneous origin of this 

 serpentine mass. 



*U. S. G. S., Monograph xxix. p. 81, and pp. 99-101. 



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