SERPEXTINIZATION OP DIKE ROCKS. 



389 



largely converted into serpentine, except in those portions where the rock is 

 nearly pure pyroxene, and seems to answer very well to Professor G. H. 

 Williams' rock pyroxenite, as set forth in an article on the " Non-feldspathic 

 rocks of Maryland,"* in which he restricts the term to igneous masses com- 

 posed only of pyroxene. Indeed the area, as a whole, has striking points of 

 similarity to certain portions of the large gabbro-peridotite area near Balti- 

 more so ably described by Professor Williams,t a certain small area, ap- 

 parently genetically connected with the dike, containing plagioclase and 

 forming a gabbro, other portions being unquestionable peridotites, while, as 

 before stated, some portions of the area contain little else than pyroxene. 

 The area is, as a whole, quite dike-like, the length being about five miles 

 and the average width less than half a mile. 



The dike has largely been converted into serpentine; but at several points 

 in the serpentinized portions microscopic sections show remains of olivines 

 and pyroxenes. It is likely that much of the rock, now serpentine, forming 

 the bulk of the area originally contained considerable olivine, those portions 

 composed of nearly pure pyroxene better resisting decomposing influences. 



Both orthorhombic and monoclinic pyroxene occur in the peridotite, the 

 latter in much larger amount. The rhombic pyroxene is slightly pleochroic, 

 the A ray being perceptibly reddish. It is doubtless bronzite. The mono- 

 clinic pyroxene is mostly or entirely diallage. 



In specimen number 322 the large dial lages contain some granules of per- 

 fectly fresh olivine, and there is also considerable olivine between the large 

 pyroxenes. This rock is practically a Iherzolite. 



Number 242 contains little or no olivine but both rhombic and mono- 

 clinic pyroxene, and answers very well to Professor Williams' websterite.J 

 That portion of the serpentine dike two miles west of the main peak, indi- 

 cated by inclined lines on the map forming plate 15, is nearly all pyroxenite. 



In the supplement to this paper (page 406) Dr. Melville gives an analysis 

 of pyroxenite (specimen number 242). It does not differ greatly in compo- 

 sition from the Avebsterite of Williams. § 



The following rocks represent decomposition products of the peridotite- 

 pyroxenite dike (see analyses by Dr. Melville in supplement) : 176 and 181, 

 series II (page 406); 222 and 223, series III (page 408) ; and 235 and 239, 

 series IV (page 409). 



All of these serpentinous products contain less silica and less magnesia 

 than the formula for serpentine given by E. S. Dana|| requires. They also 

 contain alumina, which would be expected in a serpentine derived from a 

 peridotite containing diallage and bronzite. That pure serpentines are 



* American Geologist, vol. VI, 1890, pp. 35-49. 



fBulletin 28, 1880, U. S. Geol. Survey. 



t American Geologist, vol. VI, 1890, p. 44. 



gibid., p. 41. 



II Text Book of Mineralogy, 1884, p. 350. 



