1 8 NEW YORK STATE MUSEUM 



As to the external form of the serpentine, it is that of diopside 

 grains, when it simply replaces the latter, and these are commonly 

 rounded or irregular. In cracks, the serpentine, of course, conforms 

 to the opening and, thus, appears in straight or sinuous veins or as a 

 network of these. As a replacement of calcite, serpentine some- 

 times develops along the cleavage of the latter (plate 2, figures i and 

 2) but, more commonly, is bounded by a series of intersecting 

 curves which may, however, be very jagged in detail (plate 3, figures 

 I and 2). When present in small amount, it occurs in isolated 

 globular, reniform or irregularly rounded masses (plate 4, figure i), 

 sometimes connected by veins (plate 4, figure 2) and as these spread 

 and coalesce, it increases up to a point when there is merely a little 

 calcite filling the interstices between adjacent masses of serpentine 

 (plate 5, figures i and 2). More rarely, serpentine clearly replaces 

 minerals other than calcite as, for instance, barite (plate 6, figure i). 



Between the two types of serpentine, no sharp line can be drawn, 

 and there is no reason why a mass of serpentine resulting from the 

 alteration of diopside in place, should not grow larger by the addition 

 of more serpentine from circulating solutions. As a matter of fact, 

 in many cases, grains of pseudomorphous serpentine have been 

 enlarged by the addition of replacement serpentine which, with its 

 characteristic scalloped margin, lies outside the smoothly curved 

 margin of the pseudomorphous grain. Examples of this relation 

 are shown in plate 6, figure 2 and plate 7, figure i. The illustrations 

 do not show the line of separation between the two types of ser- 

 pentine but its smooth curve is indicated by the form of the contact 

 between pseudomorphous serpentine and ' sulphides as contrasted 

 with the scalloped edges of replacement serpentine. 



The practical distinction between the two genetic varieties is 

 difficult, however, save in typical cases, which are the exception 

 rather than the rule. 



Talc is much less abundant in the ores and wall rocks than ser- 

 pentine and, being easily confused with white mica, is more difficult 

 to identify. Like serpentine, it appears in two forms as plates or 

 tufts in calcite and as an alteration of the anhydrous silicates. In 

 the former, it has evidently been deposited from circulating 'solutions 

 and has replaced an equivalent amount of calcite. In the latter, 

 the original silicate, being unstable under the conditions present, 

 broke down in a manner closely analogous to that leading to the 

 formation of serpentine. So far as rather limited data indicate, 

 the latter mineral is formed very largely from diopside, and the talc 



