NO. 1801. MISCELLANEOUS MINERALOGICAL PAPERS— POGUE. 573 



orientation, as shown by crystal outline, position of optic axes, and 

 direction of the three rays of a percussion figure produced when a 

 cleavage plate is struck by a sharp-pointed instrument. 



Thick crystals of intergrown micas are much rarer than the inter- 

 grown sheets, with which the above citations have chiefly to do. Of 

 such a kind is an example from Ottawa, Canada, in the Shepard col- 

 lection of the U. S. National Museum (Cat. No. 195). Figure 4, plate 

 63, is a clinographic projection of this crystal in natural size and 

 development, accompanied by a plan of both its upper and lower basal 

 planes. The stippled portion is biotite; the white, phlogopite. The 

 form is that of biotite and the faces c(OOl), 0(112), //(111), and 

 5(010) are well developed and easily identifiable by contact measure- 

 ment. The angles for phlogopite are so nearly identical with biotite 

 that probably the same faces are represented on the included phlogo- 

 pite crystal. It is notable that of the comparatively large number of 

 faces capable of occurring on biotite, only those are found which are 

 included among the much fewer known forms of phlogopite. The 

 biotite has a very small optic angle ; the phlogopite a still smaller one, 

 appearing almost uniaxial. The two optic planes, how^ever, may be 

 seen to be parallel, both lying in the plane of symmetry (010) and 

 parallel to the h faces. 



Chiefly within the phlogopite, but to a less extent running irreg- 

 ularly out into the biotite, there are visible under the microscope 

 multitudinous inclusions of minute needles, most of which cross at 

 angles of 60°, forming a triangular pattern. A less number, however, 

 are arranged at right angles to the three main sets, and others are at 

 random. These needles are of extreme thinness and so interleaved in 

 the basal cleavage of the mica that successive ones may be brought 

 into focus through the vertical extent of the thinnest obtainable 

 cleavage sheet. 



For their optical study and determination they were isolated from 

 the mica by attacking the latter with hydrofluoric and heating the 

 partly decomposed residue with strong nitric acid, whereby the 

 needles were obtained floating in the liquid, completely freed from 

 their host, and could be transferred by proper manipulation to a 

 shde. 



The inclusions were themselves unaffected by such violent treat- 

 ment,'^ and when in suspension their uncorroded faces reflected the 

 Ught in prismatic colors, due to the excessive thinness of the reflecting 

 surfaces. 



Viewed under high magnification, the needles are seen to have a 

 varied habit, occurring principally as long, slender laths with square 

 or pointed terminations ; but also in the form of rhombs, rectangles, 

 distorted six-sided plates, wedges, knee-shaped twins, and so on (see 



o These were resistent likewise to the action of sulphuric acid. 



