408 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



" composed of angular fragments of porphyry and compact feldspar, re-united 

 by a paste of the same materials, which is itself also porphyritic. Hence it 

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

 pact feldspar and porphyry) which was subsequently broken up, either by the 

 mechanical agency of water, or the mechanico-chemical agency of heat, redis- 

 solviug and mingling the materials." 



Of the geological position he remarks : — 



" Am I asked whether the porphyry of Massachusetts belongs to the Primi- 

 tive, Transition, or Secondary Class ? I reply that it belongs to none of them, 

 Ijut is a member of a series of rocks consisting of granite, sienite, porphyry, 

 and greenstone, which have been protruded through or among the stratified 



rocks, subsequent to their deposition The mere existence of these 



rocks, therefore, among those of any particular stratified class, does not prove 

 thai they were produced at the same epoch ; it rather proves that the imstrati- 



fied rock was of subsequent production I have never met with an 



instance in which this porphyry was exhibited in juxtaposition with any strati- 

 fied rock : except as already remarked, the compact feldspar succeeds to the 

 graywacke as an older rock and gradually passes into porphyry. This por- 

 phyry, however, is associated, both on the north and south of Boston, with 

 sienite ; and in all cases, so far as I have observed, the porphyry lies above the 

 sienite, and there is a gradual transition between the two rocks." 



The compact feldspar was considered to have been derived from the 

 melting of common feldspar or albite, in connection with other min- 

 erals. He remarks as follows in regard to this compact feldspar : — 



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

 doubt, when I often see specimens that have not entirely lost their foliated 

 structure, being intermediate between the two minerals It is not un- 

 common to meet with specimens of porphyry that exhibit traces of an origi- 

 nally slaty structure in all or a part of the materials composing it. This clearly 

 points us to a slaty rock as the source from which porphyry was derived. And 

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

 flinty slate, &c. are scattered through the mass as if partly melted down ; very 

 much as fragments appear in the slag of a furnace. They seem to be all but 

 incorporated with the paste, and the whole mass presents an appearance of a 

 more perfect chemical union than any rock resulting from aqueous agency 



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



this rock into sienite, without any apparent change of ingredients, seems to 

 indicate that the peculiarities of porphyry did not result chiefly from the 

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



Under the term syenite President Hitchcock included "all the varie- 

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

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



