508 VARIETIES. 



the so-called magnesian slate with a small amount of silica and much potassa, with the dysyntribite of 

 Professor Shepard, which forms beds in the Laurentian rocks of Northern New York, and resembles serpen- 

 tine. You will find analysis by Brush in Silliuian's Journal, second series, Vol. XXVI, p. 68, as also 

 analyses of what I call parophite. It is a shale of the Hudson River group, locally altered by trap, and 

 changed into what seemed a serpentine, whence the name. In Brush's paper, analysis 3, p. 68, is of the 

 unaltered earthy shale, which further on passes into a green, translucent, unctuous mineral analyses 1 

 and 2. (See also my Report, as cited by Brush.) We can scarcely doubt that the mineral from the Lau- 

 rentian of Northern New York, where the same matter occurs crystallized as gieseckite (see Brush), is 

 identical with you rmagnesian slate and my parophite and unaltered shale since they agree in their pe- 

 culiar composition, so different from ordinary shales and slates. These matters I regard as perhaps a sort 

 of kaolin which has fixed a portion of the potash." 



I learn, also, from Mr. Hunt, that he has found specimens of real talcose (magnesian) schist among the 

 group of rocks now under discussion. It seems, then, that a protracted, minute, chemical examination is 

 yet needed before the members of this group of rocks can be satisfactorily named. 



We will describe the varieties belonging to this formation under two heads ; first, the 

 varieties of talcose schist; and secondly, the characters of all the associated or interstrati- 

 fied rocks except serpentine, steatite and azoic limestone. 



1. Schistose Talc. This variety is more or less distinctly foliated, and varies in color from blackish-green 

 to very light green or greenish- white. It is the least abundant of any of the varieties. 



2. Steatite. This is properly a variety of this formation, but as it occurs in considerable quantities, and 

 is of great consequence, both theoretically and economically, we describe it and all its varieties under a 

 distinct head. 



3. Chlorite Schist. Sometimes this variety is foliated, and of a dark-green color ; and in such cases it is 

 distinguished from talc externally, chiefly by its darker color. In composition chlorite is a hydrous silicate 

 of alumina and magnesia, while talc is a hydrous silicate of magnesia. In structure it approaches what in 

 popular language is called slate. In this case it probably owes its slaty structure to a small proportion of 

 quartz which it contains. Belts of this variety are quite common. For instance, in the west part of Berkshire, 

 running down to Essex, and perhaps to the extreme south end of the west range ; another, running north 

 through the east part of Hydepark ; and a third, parallel to this, in the east part of Wolcott. Other local 

 deposits are noticed upon the catalogue, e. g., in the towns of Johnson, east part, Granby, Middlesex, west 

 part, and Bethel. 



4. Serpentine. This, like the steatite, is of so much consequence that we describe it under a separate head. 



5. Quartz and Talc. This is the variety in which magnesia is really present. The talc is usually scaly, 

 and the quartz arenaceous. Sometimes, however, the latter is coarsely granular or hyaline. But it is with 

 great difficulty that any distinction can be drawn between this and the following variety. We know of no 

 means of distinguishing the two except by chemical analysis. And the real magnesian schist seems to be 

 the least common of the two. 



6. Talcite (?) and Quartz. By this term we include the non-magnesian schists. Regarding it as the 

 common rock of the formation, we will mention some of its minor varieties. Of these there are two general 

 kinds : those with a very unctuous feel, and those that are harsh and rough, though scarcely differing in 

 appearance from the softer specimens. There is first a fine-grained, light green colored, rather slaty rock, 

 very soft, so that thin specimens can easily be picked into pieces by the fingers, and exceedingly unctuous. 

 There is an immense amount of this rock in the eastern part of the middle range in the northern half of 

 the State. Associated with it in alternate layers is a rock distinguishable from it only by superior hardness. 

 Then there is a compact schist, with quite evenly separated layers, of an average thickness of less than an 

 inch, rather fine-grained, but less unctuous. This terminates in a real slate, such as can be distinguished 

 from clay slate only by the absence of the argillaceous odor. A fine locality of this, and of its junction 



