4S6 R. W. ELLS MICA DEPOSITS OF THE OTTAWA DISTRICT. 



Mode of Occurrence of the Micas. — From recent observations it may be 

 stated that the merchantable micas of the district occur under six prin- 

 cipal conditions, thus : 



1. In pyroxene intrusive rocks which either cut directly across the 

 strike of greyish or other colored gneisses or are intruded along the line 

 of stratification. Some of these deposits have been worked downward 

 along the contact with the gneiss, where the mica is most generally found, 

 for 250 feet, as at the Lake Girard mine, and irregular masses of pink cal- 

 cite are abundant. In certain places apatite crystals occur associated with 

 the mica, but at other times these are apparently wanting. As in the case 

 of apatite deposits, mica occurring in this condition would apparently be 

 found at almost any workable depth. 

 ■ 2. Id pyroxene rocks near the contact of cross-dikes of diorite or fel- 

 spar, the action of which on the pyroxene has led to the formation of both 

 mica and apatite. Numerous instances of this mode of occurrence are 

 found, both in the mines of apatite and mica, the deposits of the latter in 

 certain areas being quite extensive and the crystals of large size. 



3. In pyroxene rock itself,* distinct from the contact with the gneiss. 

 In these cases the mica crystals, often of large size but frequently crushed 

 or broken, apparently follow certain lines of faults or fracture. Some of 

 these deposits can be traced for several yards, but for the most part are 

 pockety. Some of these pyroxene masses are very extensive, as in the 

 case of the Cascade mine on the Gatineau river and elsewhere in the 

 vicinit3^ In these cases calcite is rarely seen and apatite is almost 

 entirely absent. When cut by cross-dikes conditions for the occurrence 

 of mica or apatite should be very favorable. 



4. Dikes of pyroxene, often large, cutting limestone through which 

 subsequent dikes of diorite or felspar have intruded as in Hincks town- 

 ship. The crystals occurring in the pyroxene near to the felspar dikes 

 are often of large size and of dark color, resembling in this respect a 

 biotite mica-. 



The mica found under the conditions stated above in one, two, three 

 and four is all amber-colored and of the variety known as phlogopite or 

 magnesia mica. 



5. In felspathic-quartzose rocks which constitute dikes often of ver}^ 

 large size, cutting red and greyish gneiss, as at Villeneuve and Venosta. 

 These are distinct from the smaller veins of pegmatite which occur fre- 

 quently in the gneiss as the anorthosite areas are approached. In this 

 case the mica is muscovite or potash mica and is invariably found in 

 that portion of the dike near the contact with the gneiss. The crystals 

 frequently are of large size and white in color, associated with crystals 

 of tourmaline, garnet, etcetera, but v/ith no apatite, unless pyroxene is 

 also present. 



