Varieties of Mica Slate. 



327 



3. Passing into Talcose Slate. In most cases the mica slate, un- 

 der this variety, takes into its composition scales of greenish talc. 

 But sometimes, I apprehend, the mica becomes tender, almost losing 

 its elasticity, and very much resembling talc, from which it is scarce- 

 ly possible to distinguish it. When the talc predominates, especially 

 to the exclusion of the mica, the rock then becomes talcose slate. 

 (Nos. 637 to 641.) 



4. Amphibolic and Garnetiferous Mica Slate. This variety takes 

 into its composition in large proportion, hornblende or garnets; usu- 

 ally both. From the fact that those minerals are commonly found 

 together, I have made only a single variety include them both. 

 (Nos. 642 to 645.) 



5. Staurotidiferous Mica Slate. In this rock the mica is in very 

 fine scales, and it has the general aspect of argillaceous slate ; except 

 that when the strata are viewed edgewise, they exhibit a striped ap- 

 pearance in consequence of numerous layers of staurotide, which 

 appear to be coextensive with the layers of the rock. I should not 

 have regarded this mineral as of importance enough to constitute a 

 distinct variety of mica slate, did I not know that extensive ledges, 

 like the rock just described, extend nearly across the whole of Mas- 

 sachusetts, through the towns of Norwich, Chesterfield, Goshen, 

 Hawley, and Heath ; and on the east side of Connecticut river, it has 

 been traced, with some interruptions, from near Long Island Sound 

 to Franconia, New Hampshire, a distance of nearly 200 miles. 

 Where it crosses Massachusetts, however, it is but imperfectly devel- 

 oped. (No. 646.) 



I wish here tojremark, that when I coin a new term to prefix to a 

 variety of rock, it is rather for the sake of giving a laconic definition, 

 than in the wish or expectation that it will become a permanent name 

 for the rock. Indeed, mere varieties need no distinct names, except 

 when an attempt is made to give a logical account of a formation. 



6. Spangled Mica Slate. The basis of this singular rock is the 

 same as in the last variety ; and the two are associated in Chester- 

 field, Goshen, Plainfield, &c. Through this base are disseminated 

 numerous thin foliated plates of a deep brown color, resembling mi- 

 ca 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 di- 

 rection, and the surfaces of the plates in the same or in parallel planes ; 



