EASTERN MASSACHUSETTS. 



431 



Mr. Dillcrj on tbc otlxer liaiid, allowed tliat the fclsitcs were not 

 cxpoaed within Kome eight huudrcJ and fifty feet of the stratified rocks, 

 and that when last seen they were as porphyritic as at any other point. 

 The accuracy of his worlc lias not been impeached by any one ; nor is it 

 easy to sec how his conclusions can fail of acceptance on the part of 

 those Aimihar with the lithological characters of eruptive I'ocks. 



Mr. Diller found the granite breaking through and holding fragments 

 of the stratified rocks. The felsite was seen always to cut the granite, 

 but the reverse could not be found. Splinters in curved and cresceut- 

 shaped forms were found in the rhyolitic (fclsitic) ash, and in their 

 forms were identical with those so commonly occurring in tlie rliyolitia 

 ashes of the West. These forms, as would naturally occur in such old 

 glasses, were replaced by silicious material, forming pseudomorphs. 

 Tlie microscopic characters thus sustained the relations indicated by 

 the work in the field. Mr. Diller clearly showed that, while part of the 

 ashy material w^as earlier than one felsite at least, since it was cut by 

 dilvcs of it, it had been in part worked over by water and stratified. 

 'J'liis water-deposited ash passed into conglomerate and sandstone in 

 places, as is natural in any detrital material having that origin. The 

 ash when consolidated closely resembled the parent rock, and led many 

 observers to think that here the transition between sedimentary rocks 

 and true felaites occurred. (Proc. Bost. Soc. ISTat. IF ,L, 1880, XX., 

 pp. 3o5~3G8; Bulk Mus. Comp. Zouh, 1881, YIL, pp. lGr)"180.) 



The present writers examined from time to time, in company with 

 Mr. Dillei', parts of the region he was engaged in investigating, as well 

 as his microscopic sections, and are thus able from personal knowledge 

 to testify to tlie accuracy of his work. In our opinion, there can be no 

 doubt that the felsitic I'ocks of Eastern Massachusetts sitow all the 

 characters that a modern volcanic rhyolitic district would, if situated on 

 a sea-shore, and afterwards subjected to the ordinary denuding and 

 metamorpliic agencies. In places, in the field, there can be but little 

 dilTerence perceived between the modern and ancient forms. 



In his Contributions to the Geology of Eastern IMassachusetts, 1880, 

 on account of some objections made by Dr. Hunt; Mr. Crosby called the 

 Noriau series of his former paper, the '* Naugus Head series." The ac- 

 count of its generally eruptive character is given nearly as before, and 

 is as follows ([>p. 18-22) :■ — 



" That this series of pyroxeuic and feldspathic rocks, with its asf^ociated 

 minerals, — which is soinetiuies stnttified, oftcner eniptive, iVccpiciitly vt.ay 

 coarsely crystallbie, and always quartzless, — is distinct from aiiylhiiig ou- 



I 



