Genth.] 42 [Oct. 2, 



Serpentine. — Another very important alteration of the magnesian lime- 

 stone of Berks county is that into serpentine, which can be observed in 

 all its stages, from the pure dolomite into the pure serpentine. The latter 

 is generally of a greenish-yellow, greenish-white or yellow color, but also 

 sometimes brownish and grayish. Aragonite and calcite are frequently 

 associated and magnetite in fine grains is occasionally disseminated 

 through the mass. 



Several analyses of these serpentines have been made by my assistant, 

 Mr. Harry F. Keller, who found in those of : 



100.01 101.20 



M. E. Wadsworth, in his Lithological Studies, Cambridge, 1884, page 

 152, speaks of the serpentine of Fitztown (Fritztown), Berks Co., Pa., as 

 a product of the alteration of olivine, showing yet unaltered olivine. I can- 

 not imagine how olivine could be present in this rock and what it is which 

 he has taken for that mineral. 



The alteration of dolomite has produced, directly and indirectly, espec- 

 ially at Fritz's Island, a great variety of interesting minerals. Besides 

 serpentine and deweylite, there are grossular, vesuviante in a beautiful 

 yellow and orange-colored variety, apophyllite, chabazite, gismondite (?), 

 thomsonite, mesolite, stilbite (at Rautenbush), datolite and others. In a 

 subsequent paper I may give a fuller account of some of these. 



11. Ilmenite from Carter's Mine, N. G. Oligoclase. 



In the chrysolite rock (unnecessarily called dunite by some authors) 

 occurs a vein which contains corundum, and some cross-fissures furnish a 

 white plagioclase feldspar, others a peculiar variety of ilmenite. 



My assistant, Mr. Harry F. Keller, has made analyses of both. The 

 ilmenite has been found in two varieties (a) of a brownish-black color of a 

 somewhat purplish hue, in small masses which show an indistinct crystal- 

 line structure and basal cleavage. On the margin it becomes somewhat 

 columnar ; fracture uneven. Sp. gr. = 4.67. H = 5.5 ; the second va- 

 riety (b) occurs in rounded modules of about 1.5 to 2" in diameter, irreg- 

 ular in shape, very brittle and breaking up into small fragments of about 

 5 to 10 mm in diameter, without any regular form, with subconchoidal frac- 

 ture and tarnished with bluish and purplish colors. It much resembles 

 the so-called Schlackige Magneteisen from Unkel on the Rhine, although 

 the composition is quite different. Sp. gc = 4.68. Neither variety is 

 magnetic. 



