48 COLEMAN ON THE MICROSCOPIC 



of iron, are sometimes uiimerous, aud sleuder, transparent prisms of apatite ; iu fact, 

 almost all of the substances mentioned by Zirkel,' Hussak and others. 



Orthoclase. — Of the felspars orthoclase may be described fust, though iu our rocks 

 microliue is usually more abundant. It is oue of the least satisfactory minerals to diagnose, 

 since its crystalline form is rarely evident, and the cleavage angles of 90^ so distinctive 

 for macroscopic determinations, are seldom seen in microscopic sections, and in general the 

 characters of the mineral are negative. "When badly v^^eathered, one is often in doubt 

 whether a given felspar is not a plagioclase, in vv-^hich the striations have been obliterated. 

 The orthoclase is generally flesh-colored or red, probably from the separation of ferric 

 oxide, though this is not always evident under the microscope. Near fissures, where, 

 from the decay of plants, organic matter is present in solution, deoxidation sometimes 

 takes place, and the rock is bleached grey. Inclusions similar to those of quartz are 

 found, though in smaller numbers as a rule. Cavities containing a liquid have not been 

 certainly observed by me in orthoclase from from the drift. 



Microperthite. — By far the most common inclusions in orthoclase are thin lamellœ of a 

 transparent mineral, differing optically from orthoclase, and described by Hussak and 

 others as albite. These lamellae are arranged iu a roughly parallel way, and in cross- 

 section remind oue of a shoal of slender fish. They do not swarm equally in all parts of 

 the crystal, but vary much in numbers as well as iu size. This variety of orthoclase 

 (microperthite) is very common. In a few instances combinations of orthoclase and 

 plagioclase of a different kind occur, in which an individual of each penetrates the other, 

 so that iu polarised light an intermingling of cloudy patches, with twin striations and 

 without, may be seen. (Plate II, fig. 4.) 



Microdine. — A triclinic form of the potassium felspar, microline, is rarely absent from 

 the drift granites examined by me, and in many cases it surpasses orthoclase in amount. 

 The difference between the two is best brought out by polarised light. Moderately thick 

 sections present the most gorgeously colored tartan pattern imaginable, iu which scarlet 

 and orange and blue are mixed iu an extraordinary way. The imperfect twin lamellae, 

 which are supposed to give this structure, cross nearly at right angles, and are sometimes 

 woven as warp aud woof into a singular cloth-like texture, though generally more 

 loosely put together. Sometimes the structure described is distinct at the edges, but fades 

 out to a uniform color in the middle, reminding one of the effects of strain in modifying 

 the optical properties of bodies. (Plate I, fig. 3.) Microliue is very apt to contain the 

 inclusions previously mentioned as forming microperthite. Micropegmatite, a regular 

 intergrowth of orthoclase or microcline with quartz, is seen in a few of the sections. All 

 these varieties of felspar are very apt to weather, usually becoming turbid from the 

 formation of kaolin, while in other cases a dirty, yellowish green substance of a fibrous 

 nature forms along the cleavage lines and fills patches of the crystal. Epidote in pale 

 yellowish, faintly dichroic, grains or crystals is frec[ueutly found as a product of the 

 decomposition of orthoclase. 



Plagioclase Proper. — The sodium-calcium felspars are more variable in amount than the 

 the potassium felspars, aud, when not too much weathered, are readily distinguished by 



' Zirkel, Mikr. Besoh. der Mineralien und Gesteine, 1873, pp. 64, etc. ; Rosenbusch, Mikr. Phys. der petro- 

 graphisch-wichtigen Mineralien, 1873, pp. 222, etc. ; Eugen Hussak, Gesteinbildenden Mineralien, 1885, pp- 105, etc. 



