THE CHESTER AMPHIBOLITE AND SERPENTINES. 101 



(e) The abundant grains of a brightly pohirizing, granuhir mineral 

 seem to be sometimes epidote inherited from the amj)hil)()litc', sometimes 

 titanite formed around gi-ains of menaccanite which have wholly disap- 

 peared, while no trace of olivine can be detected. 



The stratigraphical indications are thus reinforced by the lithological 

 study, and the conclusion is rendered probable that these serpentines are in 

 large part derived from the amphibolites with which they are associated. 

 The lack of any direct proof of the presence of olivine is, however, very 

 far from proof that it was wholly absent from these beds. Indeed, this 

 mineral is so closely connected with the formation of serpentine in so many 

 cases that one may suspect its former presence here, and the observations 

 of Kalkowsky ^ on the presence of olivine in hornblende-schists led me to 

 search for it in the schists adjacent to these beds, but without avail. These 

 rocks plainly also resemble Von Drasche's ^ serpentine-like rocks (now 

 called antigorite-serpentines), and can be easily distinguished from the 

 olivine-serpentines. I have not, however, found any certain trace of 

 diallage in them, and the minerals which are present separate them quite 

 distinctly from those described li}' him. 



OLIVINE- AND KNSTATITE-SERPENTINE. 



7. ''Serpentine, Chester." — XIII, No. 53, Massachusetts Survey Col- 

 lection. Rock dull-black, black when wet; many large grains of magne- 

 tite. The rock weathers superficially to carbonate. 



With lens the slide is pale-green, and shows the secondary magnetite 

 occupying planes of lamination and a fine system of joint planes nearly at 

 right angles to these and very regular. 



Under the microscope an olivine network, inclosing in one slide frag- 

 ments of unchanged olivine, in another lacking these altogether, runs 

 through the slide without being influenced at all by the lamination and 

 joint planes mentioned above. 



This network runs through a base of nonpolarizing serpentine. I am 

 not able to locate this specimen, but suspect that it comes from the base of 

 the large Middlefield-Chester bed, or from the smaller bed upon the top of 

 Nortli Mountain. 



Serpe.ntuie. — "The Crater," North Blandford. The rock of the crater 

 is easily distinguished both macroscopically and microscopically from that 



'Die Gneissformation des Enlengebirges. p 37; Tsohermaks mineral. Mittheil., 1871, p. 1. 

 ■^Ueber Serpentin und Serpentiniihnliche Gesteine: Tschermaks mineral. Mittbi'il., 1871, p. 1. 



