J. F. Kertvp — Diorite Dyke, Orange Co., N~. Y. 



Aet, XXVIII. — A Diorite Dike at Forest of Dean, Orange 

 County, N. Y. ; by J. F. Kemp. 



Eecent workings in the Forest of Dean magnetite mine, 

 in Orange Co., N. T., prove it to be intersected diagonally by 

 a dike of diorite. This rock was referred to in the Report of 

 the New Jersey Survey for 1886, p. 107, but not until the last 

 summer was the writer able to take the dimensions under- 

 ground. The dike, about six feet in width, intersects the mine 

 workings in the western branch at an angle of 30°, runs in 

 unbroken width sixty feet across, and disappears in the foot- 

 wall. Microscopic examination of a series of sections along 

 its length proves it to be a very typical diorite quite similar to 

 those described by Hawes* from Compton Falls and by Har- 

 ringtonf from the neighborhood of Montreal. The rock is dark 

 gray in color, very fine grained, the component crystals being 

 too small to be distinguished macroscopically. The spec, 

 gravity varies from 2 '925 to 2 "974. 



Under the microscope it is found to consist of crystals of 

 plagioclase, hornblende and magnetite, together with certain 

 alteration products of the first two. The hornblende is of the 

 ordinary brown type, generally in well developed crystals show- 

 ing the prism and pinacoidal faces. It is of rather light brown 

 color, not remarkably pleochroic c = h^>a. The individual crystals 

 vary from 01 mm to 3 mm . In the more altered portions of the 

 dike the hornblende is changed to a greenish mineral resemb- 

 ling chlorite, with threads and fernlike aggregates of secondary 

 magnetite penetrating it. These thread-like aggregates are 

 exceedingly minute, not over -2^- mm in breadth, whereas the 

 original magnetite is in isolated angular masses, seldom show- 

 ing octahedral outlines, tV" "5V mm ^ n diameter. The magnetite 

 is free from indications of titanium. The plagioclase is in rod- 

 shaped crystals averaging , l mm by 0"3 mm of irregular outline. 

 Acicular inclusions probably apatite are not infrequent. The 

 extreme smallness of the feldspar crystals made any attempts 

 at separation with~the heavy solutions unsuccessful. Calcite 

 and quartz appear as secondary products, corroborating Eosen- 

 busch's general statement in regard to the Lamprophyr group 

 of the dike rocks. 



Compared with slides of the Campton Falls dike, the feld- 

 spar and magnetite are noticeably more abundant, the horn- 

 blende less so, at the same time the crystals of the last named 

 mineral are less elongated and smaller but much better devel- 

 oped. Compared with the Montreal dike much the same may 



* This Journal, III, vol. xvii, p. 147. f Geol. Survey Canada, 1877-78, 439. 



