276 PROCEEPINGS OF THE ACADEMY OF [1880. 



March 24, 1879. 



Some Microscopic Enclosures in Mica. — Mr. Theo. D. Rand 

 described, and exhibited under the microscope, certain crystals, 

 etc., included in mica, chiefly from Swain's quarry, Chester Co. Pa. 



Of these, the magnetite dendritic markings, and similar mark- 

 ings of red and brown colors, apparent!}' due to oxidation of the 

 magnetite, are most common and best known. Besides these the 

 following occur : — 



Hexagonal crystals, black and opaque ; angles, 60° and 120°. In 

 the form of the crystal in this description, the form of the section 

 exhibited under the microscope is intended. A similar crj'stal, 

 brown in color, perhaps the same substance, translucent ; probably 

 biotite or lepidomelane. 



Hexagonal or rhombic crj- stals of a bright red color, sometimes 

 with the angles modified; angles (;0° and 120°. There are some 

 specimens which indicate the change of the black into the red 

 rhombs. One of the red rhombs contained a black crystal, with 

 faces parallel to those of the red, and one, a very symmetrical and 

 simple crystal, from near Newtown Square, Delaware Co., Pa., 

 was black for about one-fourth its length, the remainder red. 



Rhombic crystals, polarizing light, giving very brilliant colors. 

 At first this was supposed to be due to films of the miea itself, but 

 the regularity and brilliancy of the rhombs, compared with the 

 mica, and their angles, seem to render this more than doubtful, the 

 angles being between 73^° and 78°. They are almost universally 

 accompanied by, and in contact with, the red or black rhombs, 

 and generally both. 



Quartz cr3'stals, generally flattened, sometimes very minute, 

 sometimes large enough for the crystallization to be seen with the 

 naked eye ; generally masses of crystals, showing distinct crj- stal- 

 lization on the edges only, occasionally separate doubly terminated 

 prisms. Some of the specimens with polarized light are ver^^ 

 beautiful. 



A substance usually presenting the form of disks, jL inch and 

 less in diameter, showing, with polarized light, a radiation from 

 the centre, and a change of brilliant colors as the analyzer is 

 rotated. Apparently the same material occurs in acicular crystals, 

 often twinned at 60° and 120^, in a plumose form, and in a form 

 closely resembling a section of agate across the layers. Some of 

 these disks appear to be strictly a radiation of acicular crystals 

 from a centre, others to be made up of three or more oval masses ; 

 sometimes the latter are separate, or joined two, three, four, or 

 six together, showing apparent twinning at 60° and 120° ; these 

 oval masses, with polarized light, take each a single tint at a time. 

 This material was found also in mica from near Newtown Square, 



