1875.] ' 4:1 i [Frazer. 



With the Lieberkuha reflector the fragments of magnetite assume a 

 partially metallic lustre. 



With one Nicol's prism there is a faint appearance of dichroism in 

 some isolated spirts of some of the pyroxene crystals but in general there 

 is no change. 



Between two Mcol's prisms the pyroxene changes from green to pink 

 (sometimes giving a transient spot of deep purple), and the irregular rifts 

 in its mass are more plainly visible. 



The Labradorite changes abru.ptly along the planes of twinning to light 

 brown and pale greenish-blue from white. The striation is very appa- 

 rent and polarization is usually complementary in two or three sections 

 of the single blade. 



The magnetite of course remains unchanged. 



Between Nicol's prisms and magniiied 275 diameters the outlines of the 

 constituent crystals of this rock are very sharp, and the pyroxene in par- 

 ticular shows very brilliant shades of purple and green. 



The cleavage is quite apparent, and the whole rock seems but little 

 altered. 



DOLERITE FROM LOGAN*S ShAFT, 1 MiLE JST. OP'DlLLSBURG. 



This slide resembles the others but is less decomposed and compounded 

 of finer crystals than the others. It exhibits Labradorite, pyroxene and 

 magnetite, besides acicular crystals which appear to ba apatite. 



Under 275 diameters the Labradorite and pyroxene have a rough ap- 

 pearance, as if covered with little bubbles, due perhaps, to incipient de- 

 composition. A number of small needle-like apatite crystals are scat- 

 tered through the mass. 



The greater part of the Labradorite (which is twinned as usual) lacks 

 sharpness of outline. 



The photographs and zinc plates from the photo-zincograph process have 

 been prepai-ed by Mr. Anthony Wenderoth, of this city, to whom great 

 credit is due for his skill in overcoming what have been hitherto co asidered 

 insuperable dilficulties. In the present state of photography it is im- 

 possible to make a picture from nature of the constituents of a complex 

 rock of this kind, and at the same time to preserve the identity of each 

 to the eye. Indeed the outlines of the separate minerals will blend more 

 or less into each other when the colors are such as wrill affect the sen- 

 sitized plate imperfectly. Another drawback is that yellow and red min- 

 erd.ls photograph black, and the former being one element of the color of 

 many pyroxenes, the black spots, which should indicate magnetite, are 

 sometimes extended out of all reason, when the two last mentioned min- 

 erals occur together. Another evil is that the same mineral may, by rea- 

 son of slightly diftering thicknesses in different parts of the slide, assume 

 totally different colors. And still another, is that part of the stippled ef- 

 fect is often due to the necessities of the process. Yet in spite of these 

 disadvantages, some of which at least experience and patience will enable 



