KEFORT (iF THE STATE GEOLOGIST. . 25^ 



sharp angled crystal, and in one of tlie greenish centers polarizes at an 

 angle of 34° with c, and the brown border at 42°. 



Rounded colorless spots full of round ore grains, seem to be minute 

 inclosures of the contact rock described above. Tliey give aggregate 

 polarization. 



The large hornblendes, under the microscope, are very fresh, and 

 have a deeper colored border where they have been corroded. These 

 remnants are often included, wholly or partly, in the large fresh 

 pyroxenes, but without crystalline orientation. In one curious case a 

 large pyroxene, cut at about right angles to the prism, has one quadrant 

 of its surface replaced b}^ the fragment of a liornblende crystal, whose 

 outside faces very nearly continue the proper boundary of the pyroxene 

 crystal, while its inside face, that is, the face by which it is grown 

 together with the pyroxene, is a fracture. Its outside faces, moreover,, 

 have the deeper resorption color, while this is lacking on the inside 

 face. There is an entire lack of orientation, the vertical «,xis of the 

 hornblende about coinciding with one of the horizontal axes of 

 the pyroxene. The pyroxenes are thus plainl}^ later than the horn- 

 blendes, and a second, much smaller generation of pyroxenes occurs in 

 well formed elongate crj^stals surrounded by a he^vy borcVr of black 

 grains. 



The rock shows distinctly an interstitial amorphous ground mass,, 

 full of minute, short, straight, brightly polarizing rods. These may 

 probably be pyroxene, and a few larger but still minute rods, raveled 

 at the ends, may also be of the same character. They do not show 

 twin striation and have too bright polarization colors to be plagioclase. 

 The ground is so full of the shapeless grains of black ore, that, in the 

 thick slides studied, the presence of plagioclase could not be made 

 certain. The rock is thus almost a pure pyroxenite. The magnetite 

 scattered through the mass is surrounded by a broad border of deep 

 red color where the glassy ground mass had dissolved the iron in part 

 and become ferruginous. This is not the case in the colorless inclosures 

 mentioned above. 



Some portions of this ground mass polarize in broad irregular patches 

 with bluish colors like nepheline, but these patches are not to be dis- 

 tinguished from the rest in ordinary light. 



Specimen " X," marked " Fragments of Limestone," is a large piece 

 of dark unaltered Trenton limestone,* with white crinoid stems, and the 



* [Note. — This limestone; away from the contact surfaces with the eruptive, is exceedingly 

 tough, and though traces of fossils are plainly evident, identifiable forms are to be seen only 

 where the rock has weathered to a thin and very soft argillaceous crust. The following species 

 will serve to determ^ine the geological age of the rock: Calymene senaria, Courad, Strophomena 

 suhtenta, Conrad, Plectambonites sericea, Sowerby. J. M. Clarke.] 



