THE CHESTER AMPHIBOLITE AND SERPENTINES. 155 



to be an igneous rock, rather than the olivine bed. The tourniaHne is, as 

 it contains much magnetite, a very late-formed mineral in the fringe rock, 

 which is itself a late product of metamorphi.sm. This association of tlie 

 emery bed with limestone may point to a similarity of origin for this bed 

 and the beds of Naxos, which are thin sheets of emery in crystalline lime- 

 stone associated with mica-schist and granite. 



8. The (jreat extent and constant horizon of the series a proof of its sedi- 

 mentary origin. — Above and below the Chester amphibolite series are highly 

 tilted sericite-schists of great thickness, often veiy quartzose, which were 

 doubtless once sandstones, of about the same age as the Berkshire and 

 Greylock schists. 



The Chester series is conformable with these, and where it loops up to 

 the north it forms a compressed syncline, pitching to the north, and east of 

 this another, more open syncline, pitching- southerh'. It has the aspect of a 

 sedimentary series, and not that of an intrusive body, and its complexity 

 and great length indicate its origin by sedimentation. I have followed it 

 more than GO miles in Massachusetts and found it an almost miiuterrupted 

 band of amphibolite, often breaking up into several beds, with thin inter- 

 posed sericite-schists; and it extends a great distance north and south, 

 across Vermont and Connecticut, and can be followed far east into Worces- 

 ter County at a constant horizon. 



I conclude, then, that the Chester series represents an eastward ex])an- 

 sion of the limestones of the Silurian sea which deposited the Stockbridge 

 and Bellowspipe limestone in Berkshire, and that it may be tentatively 

 paralleled with the latter bed. Some very basic igneous rocks may have 

 been associated with it in this, its shoreward, extension, and along its central 

 part its upper surface was, at a later time, x-eplaced locally by limouite. 



The Bolton limestone in Worcester County occupied about the same 

 horizon, and has in places changed into actinolite-schists and developed in 

 large quantity boltonite, which is almost a variety of olivine, and is in 

 places changed into black serpentine, forming- a rock exactly like the West- 

 tield quaiTy stone. The black prisms of altered boltonite scattered in the 

 white limestones reproduce on a slightly smaller scale the black rods of 

 enstatite in the latter rock. 



