408 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



-.*• 



" compo.^ied of angriLir fragments of porphyry and compact feldspar, re-united 

 by a paste of the same materials, whicli iy itself also porphyiilic Hence it 

 appears that there must have been aji original formation of these rocks (com- 

 pact i'eldspar and porpliyry) which was suhi^e(|ue]ltly broken up, either by the 



mechanical a^s^^ency of Avater, or the luechauico-chcmical agency of heat, redis- 

 solving and mingling- the materials." 



or the geological position he remarks : 



*^ Am I asked whetlu^r the poi'phyry of Massachusetts belongs to the P]'imi- 

 tive, Transitioiij or Secondary Class ? I re}>ly that it belongs to mine of them, 

 but is a me]uber of a siiries of rocks consisting of granite, sieuite, ])or])hyry, 

 and greenstone, which have been protruded through or among the sti'atilied 



rocks, su])se([ucnt to tlicir deposition The mere exiHtence uf these 



rocks, lh(.'refi)re, among those of any particnlar stratified class, does not prove 

 thiii they were })ro<Uiced at llie same epoch ; it rather proves that the unsti'ati- 



fied rock was of subs('(pient jiroduction I have never met with an. 



instance in which tlus porphyry was exhibited in jnxta])ositiou with any strati- 

 fied rock ; except as alread}^ remarked, the compact feldspar succeeds to the 

 graywacke as an older rock and gradually passes into porphyiy. Ihis })ur- 

 ])ltyry, however, is associated, both on the north and south of l>ostonj with 

 sienite ; and iu all cases, so far as I have observed, the j^orj'hijnj lies ahuvc iJce 

 sicnite, and tliere is a gradual transition between the two rocks." 



The compact feldspar was considered to hav^e been derived from the 

 melting of common felds[)ar or albitc, in connection with other min- 

 erals. He remarks as follows in regard to this compact felds})ar : 



"That it does result from this change in common, feldspar, I can jinrdly 

 doubt, when I often see specimens that have not entirely lost the.ir foliated 



structure, being intermediate between the two miiicrals It is n(»t un- 



connn(;n to meet with specimens of porphyry that exhibit traces of an origi- 

 nally shity structure in all or apart of the materials composing it. This clearly 

 j)oints us to a slaty rock as the source from winch i)or])hyry was derived. And 

 sometimes fragments of this rock, along with fragments of compact JeLlspar, 

 lUnty slate, «.tc, are scattered tlirorigh the mass as if partly melted d(jwn ; very 

 much as fragments jippear iu tlie slag of a furnace. They seem to be all but 

 incorporiit(;d with the paste, and the whole mass presents an a]ipearance of a 

 more ])errect chemical union than any rock resulting from a([ueous agency 



ever exhibits, unless it be entirely crystalline The gradual passage of 



this rock into sienite, without any apparent cliange of ingi'edieiits, st-ems to 

 indicate tliat the peculiarities of por[)hyry did not residt chiefly frnm the 

 nature of the materials employed in its production." (I. c, pp. 442-451.) 



Under the term syenite President Ilitchcock inclvuled ''all the varie- 

 ties of rock, between greenstone and porphyry on one side, and comnuni 

 granite on the other, into whoso composition hornblende enters." His 



