382 Scientific Geology. 



sequently, a slower crystalization, the particular causes which may 

 have determined the latter rather than the former effect."* 



This theory, if admitted, explains satisfactorily the approximation 

 of hornblende schist to unstratified trap rocks. For some portions of 

 the clay slate would very probably be so entirely fused as to obliter- 

 ate all marks of a stratified and schistose structure : and hence by 

 slow crystalization might result hornblende rock and primitive green- 

 stone. 



I have observed no facts in Massachusetts very decisive in respect 

 to this theory. But since granite does occur in connection with some 

 portions of our clay slate I doubt not but such may be found here. I 

 have made no examination on the subject worthy to be named. It is my 

 opinion, however, from what I know of the range of slate extending 

 into Vermont and New Hampshire from Franklin county, that in 

 those states would be the most likely places for such facts to be devel- 

 oped. The passage of graywacke slate into talco-chloritic slate, and 

 of this into hornblende slate in Smith field and Cumberland, Rhode 

 Island, has probably also a bearing upon this theory. 



The remarks that have been made in relation to porphyritic talcose 

 slate, will apply with still more force to that variety of hornblende 

 slate which is porphyritic. For in the latter case this structure is 

 more distinct and perfect than in the former. And the more I reflect 

 upon the subject, the more satisfied I am, that a porphyritic structure 

 must, in all cases, have been the result of the agency of heat. 



14. GNEISS. 



This rock occupies more of the surface in Massachusetts than any 

 other : and in all countries of much extent hitherto examined, it is one 

 of the most extensive of the formations. Quartz, mica, and feldspar, 

 are its common and essential ingredients ; though hornblende is so 

 often present, that some writers regard its claims to be considered es- 

 sential, as equal to those of the other three minerals. It is obvious, 

 therefore, that the mineralogical constitution of this rock is the same 

 as that of granite. The only difference, indeed, consists in the strati- 

 fied and slaty disposition of the gneiss. This character, however, 

 sometimes becomes very obscure ; and then it is almost impossible to 

 distinguish between the two rocks. They might, therefore, be regard- 



* System of Geology, vol. 2. p. 171. 



