18 93. J NEW YORK ACADEMY OF SCIENCES. 213 



minerals are so abundant in tbe rock there can be little doubt 

 that they are the source of the serpentine. 



Another j^hase of the limestone shows, scattered through the 

 mass, imperfect prismatic crystals, averaging about an inch in 

 length, and dark gra^' or black. On weathered surfaces these 

 crystals project from the rock surface, but are themselves much 

 decomposed. Under the microscope the mineral shows the rect- 

 angular cleavage, parallel extinction, high interference colors, 

 uniaxial figure and negative character of scapolite. The cause 

 of the dark color is seen in the presence of great quantities of 

 small black inclusions. These are of irregular shape, often 

 elongated parallel to the vertical axis of the scapolite. In some 

 cases they are quite evenly distributed through the mineral, in 

 others, are grouped in patches ; but nearly always a thin outer 

 layer of the crystal is quite free from inclusions. Similar inclu- 

 sions in scapolite are described by Rosenbusch * as consisting 

 of carbonaceous matter and this is undoubtedly true of those 

 under consideration. The same rock contains scapolite, iisually 

 in small grains, which contains no inclusions whatever. Titan- 

 ite, i^yroxene and mica are also abundant, the two latter some- 

 times intergrown. 



Other varieties of the limestone are plenty, but as their interest 

 is mineralogical rather than petrographic, their consideration 

 is beyond the scope of this paper. 



QUARTZITE. 



The Potsdam sandstone, so far as examined under the micro- 

 scope, possesses the character of a very pure quartzite. It is 

 made up of w^ell rounded grains of quartz, cemented into a 

 tough, compact mass by the deposition of secondary silica. 

 Besides quartz no mineral has been found forming complete 

 grains, though other species are not uncommon as inclusions 

 in the quartz. Of the minerals occurring in this way biotite 

 zircon, apatite and rutile are the most abundant. Liquid inclu- 

 sions are also present in great numbers, but never formed in the 

 quartz subsequent to its incorporation into this rock. 



When, as often happens, the rock is red, the color is due to 

 finely divided hematite, which forms a thin coating over the 

 quartz grains. The hematite clearly was deposited uj^on the 

 quartz before the introduction of the cement, and its appear- 

 ance suggests that it is the result of subaerial erosion, formed 

 by some such process as that outlined by Russell, f 



* Mikroskopische PhysioRraphio. 3d edition, I., p. 363. 



t Russell, I. C. The Suljaoriul Decay of Rocks, Bulletin r,2. U. S- G. S. 



