34 



the Merrimac, in Newburyport and Salisbury, appears to be in- 

 termediate between this variety and the preceding. Portions 

 of it, however, approximate to the third variety in the pro- 

 portion of hornblende they contain. 



3. The more hornblendic of the Huronian granites are usu- 

 ally fine-grained, and the increase of the hornblende is com- 

 monly attended by a diminution of the quartz ; so that the rock 

 exhibits, through the entire absence of quartz, and, probably, 

 a concomitant change from orthoclase to a triclinic feldspar, 

 frequent passages into diorite. 



This variety is subject to equal or greater variation in com- 

 position and texture than those already described, and its 

 boundaries are even more difficult to trace. In northern Lynn 

 and Saugus, between the petrosilex on the south and the typical 

 granite of Lynnfield and Peabody on the north, is a belt of this 

 rock nearly a mile wide and more than three miles long. 

 Geographically, it sometimes occurs, as in this instance, be- 

 tween petrosilex and granite ; but, as a rule, it is associated 

 with diorite, and this association is usually very intimate. This 

 diversity of petrologic relations is due to the fact that, as re- 

 gards its origin, we are dealing with more than one rock. 

 Those portions only of this variety that are associated in their 

 distribution with either the typical or euritic varieties, or with 

 petrosilex, are marked as granite on the maps ; when connected 

 with diorite alone, it has been (for reasons that will appear 

 farther on) regarded as diorite, and has not received a dis- 

 tinctive color. This rock is always finer-grained and more 

 hornblendic when accompanying diorite than when connected 

 with petrosiiex, or with other granites ; and, in the former 

 association, it rarely approximates to either the typical or 

 euritic varieties, or to petrosilex ; in fact it is doubtful if in 

 such cases the feldspar is orthoclase, plagioclase being more 

 probable, — in which event the rock becomes, of course, 

 simply a quartzose diorite. 



The Huronian granites of this region sometimes exhibit traces 

 of stratification. This structure has been observed in the fine- 



