404 



Scientific Geology. 



" geological philosophy must adopt this (proposal) sooner or later." 

 But if that geologist shrink from taking the lead in such an innova- 

 tion, well may I. The members of the unstratified class in Massa- 

 chusetts are not numerous ; and I have concluded to treat of them all 

 under four divisions, viz. greenstone, porphyry, sienite, and granite. 



There would be some advantages in treating of these rocks in an 

 ascending, instead of a descending order : that is, in beginning with 

 granite, taking sienite next, porphyry next, and greenstone last. 

 For this is the order in which in general they seem to have been pro- 

 duced. But for the sake of uniformity, and to secure some other ad- 

 vantages, I shall invert this order. 



A few words may be needful in this place, in respect to the man- 

 ner in which these rocks are represented on the map. From the in- 

 tricate manner in which the greenstone, sienite, and granite are mixed 

 together, in the vicinity of Boston, I found it impossible to give them 

 precisely their true relative space in the delineation. I therefore col- 

 ored the whole space occupied by them all, as granite ; and then, 

 having observed that as a general fact the greenstone was first met 

 with, on lines radiating from Boston, then porphyry and sienite, and 

 lastly granite, I represented these several rocks as occupying spaces 

 somewhat in the form of concentric bands. Wherever I observed these 

 rocks intermingled, however, I have endeavored to represent their 

 mixture by scattering dots and crosses somewhat promiscuously in the 

 region. This method of course can give only an approximation to 

 the truth. In the valley of the Connecticut, these rocks are scarcely 

 ever so confusedly mixed together ; and, therefore, it is only in the 

 eastern and northeastern parts of the State that such a course has 

 been adopted. The porphyry forms only two ranges, which are dis- 

 tinct ; the one on the north, and the other to the south of Boston, 

 except that a narrow strip of compact feldspar, — the base of por- 

 phyry, — is marked in the northern part of Essex county. Por- 

 phyry, however, passes by insensible gradations into sienite : but the 

 change commonly takes place in a vertical and not in a horizontal 

 direction. 



15. GREENSTONE. 



The most approved definition of this rock makes it to consist of 

 hornblende with compact and common feldspar : some add clinkstone, 

 also ; but as this substance does not occur in this part of our country, 



