THE CHESTER AMPHIHOLITE AND SEKPENTINES. 153 



been derived, ;iud will compel one to hold to the independence of the two, 

 if one will not accept their conunow derivation from a dolomite. 



G. Derivation of the bJdik, thin-fissile amphibolites from the limestones. — 

 If the steps by which the black serpentine has been traced backward in 

 the preceding- sections be valid, a goodly portion of the series for 20 miles 

 from the Westfield end, and in places its whole thickness, was ori<i;inally 

 a dolomitic limestone. Associated with this black serpentine series, in 

 gradually increasing quantity as we go north, is the black, or dark-green, 

 amphibolite, made of a matted network of actinolite needles in an albite 

 mosaic. The two sei'ies are closely joined in all this distance; they replace 

 each other along dip and strike, and if the amphibolite be an altered erup- 

 tive it must have been intruded by a kind of preestablished harmony, so 

 as to fit itself to the limestone exactly through this long distance, without 

 showing any eruptive relations to it or preserving now any eruptive rock 

 texture. 



'Therefore, as in the case of the same amphibolites which in the same 

 way attach themselves to the pre-Cambrian limestones throughout their 

 whole extent in Berkshire County (see p. 29), I conclude that these rocks 

 are, in the main, derivatives of an impure argillaceous limestone, and I 

 extend this conclusion to the long range of amphibolite which goes north 

 from Blandford across the State, interrupted only by areas of serpentine 

 derived from the alteration of the amphibolites. The residual facts which 

 point to the eruptive origin of some part of the series are the constant 

 presence of chromium and nickel in the serpentine from several localities, 

 as indicated in the table of analyses on ])age 116, and the close agTeement 

 in composition and structure of the dark amphibolites with rocks which 

 have been proved to be derived from basic eruptives. 



In relation to the first point, it is very probable that olivine-bearing 

 rocks may be present in the serpentine masses north of the Blandford and 

 Middlefield olivine occurrences, though long search in the field and the 

 study of many sections has compelled me to refer all these beds to the 

 category of translucent hornblendic serpentine. In these great areas of 

 seqientine much may have been overlooked, as sections could not be cut 

 from every part. The presence of these assumed olivine rocks may have 

 been the initiating cause of a sei^pentinizatiou which extended far beyond 

 the rock in which it started. Further, an inspection of the table of analyses 

 shows that the eustatite and sahlite, minerals which have certainly formed 



