194 MINERALOGY AND LITHOLOGY. 



granites; — therefore the micaceous rocks in our state which contain only 

 muscovite are the more quartzose varieties of schist. 



Ahiscovite Biotite Granite. The larger part of those fine-grained com- 

 pact granites that are so extensively cjuarried in our state is of this 

 kind. The Concord granite is a beautiful stone, which is fine in texture 

 and light in color. Larger twin orthoclase crystals are sparingly distrib- 

 uted through it. Thin sections show some interesting features. Finely 

 banded oligoclase crystals are common; and through some of the ortho- 

 clase crystals laminas of a triclinic feldspar are arranged in two direc- 

 tions at right angles to one another. This kind of interlamination is 

 frequently observed in free-grown crystals, in which case the laminas are 

 usually of albite ; and it is very likely that they are of albite in this case, 

 although proof of it is scarcely possible. A crystal of this kind, as seen 

 in a section of Concord granite, is represented in Fig. 6 on PI. 5. The 

 quartz is very clear, and the cavities in it are very small ; but every grain 

 is penetrated through and through with the most minute dark needles of 

 rutile, which look like the finest hairs. The larger grains of mica are not 

 crystalline in outline; but some very perfect microscopic crystals are scat- 

 tered through the quartz. Occasionally a hexagonal needle of apatite is 

 found, which, though small, is recognizable. A bit of chlorite is rarely 

 seen. The orthoclase is tolerably clear; and the structure and composi- 

 tion of the whole indicate a very durable rock. 



This Concord granite is typical of a group of granites which are re- 

 garded by Prof. Hitchcock as belonging to the stratified series which 

 forms the Montalban system. A number of thin sections have been 

 made from members of this group (called Concord granites), and the 

 variations in their microscopic characters are of an unessential nat- 

 ure. In sections of the Fitzwilliam granite, crystals of zircon are notice- 

 able, which, though very minute, are quite perfect. Some of them have 

 been represented in Fig. i on PI. 5. The crystals of rutile characterize 

 the quartz of this as of all the Concord granites. The crystals of inter- 

 laminated feldspar are not found, but others, with the very intricate bas- 

 ket-work structure that characterizes microcline, are seen. In another 

 specimen, which is cut from what is called the best quality of the Fitz- 

 william granite, a pretty microscopic structure is developed in some of 

 the grains of feldspar, which is caused by the arrangement through them 



