Economical Geology. 



•were originally applied to designate rocks very different, if not in 

 composition, yet in their geological relations. But most of the rock 

 that is generally described as sienite, is a variety of granite. This is 

 certainly the case in Massachusetts. Wherever the granite admits 

 hornblende into its composition, I have considered it as sienite ; and 

 not unfrequently the hornblende constitutes the principal ingredient ; 

 taking the place, more or less, of the quartz and mica, so as to form 

 a compound of hornblende and feldspar. This compound forms 

 some of the most beautiful varieties of sienite, though extremely 

 hard to work for architectural purposes. But not a little granite that 

 contains no hornblende goes by the name of sienite. Thus, much of 

 the Gluincy granite is wanting in hornblende ; but being almost des- 

 titute of mica, and having the close aspect of sienite, it is called in- 

 differently by either name. 



The variety in the composition, color and hardness of these rocks 

 in Massachusetts, is almost endless. The quartz and feldspar are 

 commonly white, yellowish and gray ; the latter not unfrequently 

 flesh colored : the mica is very often black, but sometimes of a silver 

 color. When the quartz prevails, the rock is easily broken, but 

 hornblende renders it tough. The predominance of feldspar gener- 

 ally gives the rock a more lively white color and renders it rather 

 easier to work. But I shall not attempt to describe particularly all 

 the varieties of these rocks that occur in the State. An inspection of 

 the specimens which I have collected, will at once give an idea of the 

 kinds obtained at the principal quarries, and of numerous other vari- 

 eties which I have met with in different localities. 



The very coarse varieties of granite, which are found in some parts 

 of the State, do by no means furnish a good building stone : indeed, 

 some of them hardly serve for common walls. Much of the granite 

 in the vicinity of Connecticut river is of this description ; as also a 

 considerable portion of the range which extends from Southborough 

 to Andover ; particularly along its northwestern limits. But most of 

 the granite in the eastern part of the State, is of so fine a texture, as 

 to answer admirably for architecture and other economical purposes. 

 Along with sienite, it extends around Boston, running in a curvilinear 

 direction at the distance of fifteen or twenty miles. From Cohasset 

 to Q,uincy, at the southern extremity of the curve, and from the end 

 of Cape Ann to Salem, on the north, the formation is most fully de- 

 veloped, and is there quarried extensively. The Gtuincy quarries are 

 probably the best and most generally known ; and few citizens of the 



