MICA SCHIST. 475 



MICA SCHIST. 



BY E. HITCHCOCK, SENIOR. 



The normal description of this rock makes it consist of alternate layers of mica and 

 quartz ; but often it passes insensibly into other schists, and the ingredients are much 

 modified. The quartz sometimes disappears, leaving only small flakes of mica and then 

 verges towards, and at length becomes clay slate. On the other hand the laminae of quartz 

 become thicker and the mica less, until quartz rock is the result. Sometimes it takes 

 other simple minerals into its composition ; as for instance talc, when it becomes talcose 

 schist, or hornblende, which at length converts it into hornblende schist, or feldspar, 

 which makes it gneiss. But the most important variety, so far as Vermont is concerned, 

 is where the rock takes into its composition carbonate of lime. It does not simply con- 

 tain interstratified beds of the lime, but the two ingredients interpenetrate each other ; 

 sometimes one predominating and sometimes the other. This we call calciferous mica 

 schist, and it so predominates in the State that we have sometimes doubted whether all 

 the mica schist that exists there was not originally of this description, and subsequently 

 deprived of its lime in some cases by metamorphic action. We shall, however, place one 

 or two bands of pure mica schist upon the map ; but the greater part is calciferous mica 

 schist. 



Lithe-logical Characters. 



The preliminary remarks that have been made will much abbreviate the descriptions which follow. 



1. Mica and Quartz. The two minerals are interlaminated, and vary in relative quantity. 



2. Arenaceous. Here the quartz is in the form of a fine, usually gray sand; the mica, however, has a 

 parallel arrangement. This is the variety used for whetstones and firestones. It is sometimes called Whet- 

 stone Slate. 



3. Argillo-micaceous. This is when the rock passes into clay slate, as above described; and often over 

 wide areas it is difficult to say whether the rock should be called mica schist or clay slate. 



4. Plumbaginous. This hardly differs in appearance from the last; for fine argillaceous matter subjected 

 to certain degrees of metamorphism, especially if containing some carbon, much resembles plumbago or 

 black lead. 



5. Amphibolic. This takes amphibole, in the form of hornblende, into its composition, in such quantity 

 as to give it a character. But it does not usually pass into hornblende schist. 



Garnets and staurotidaa sometimes in like manner impart peculiar characters to mica schist, and make 

 the varieties to be properly called garnetiferous and staurotidiferous. 



6. Spangled mica schist. The basis of this variety is usually arenaceous, and it is thus described in the 

 Final Keport on the Geology of Massachusetts, where it was first pointed out. " Through the base are 

 disseminated numerous thin foliated plates of a deep brown color, resembling mica somewhat, but almost 

 entirely destitute of elasticity, and brittle. Their length (rarely more than a quarter of an inch) is usually 

 twice as great as their breadth, and there is a decided polarity exhibited in their arrangement, that is, their 

 longer axes all lie in the same direction, and the surfaces of the plates in the same or in parallel planes; so 

 that light is reflected from many of them at once when the specimen is held in a proper position, and 

 thus a beautifully spangled appearance results. Not being confident as to the nature of this mineral, 

 I have given the rock a designation which indicates merely this obvious property. These spangles are 

 pretty uniformly diffused through the mass, and their surfaces rarely coincide with the layers of the slate." 

 page 594. 



To this description, based upon this variety of mica schist found on the east slope of Hoosac Mountains 



