404 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



Titanite is of frequent, zircon of rather rare, occun-ence. The titanite 

 is found not only inclosed, as already stated, in biotite and chlorite, but also 

 in well-rounded clastic grains which are often bordered with an opaque ore. 

 Zircon occurs in broken grains, without doubt clastic, and also in small 

 crystals which show no signs of wear. These last were probably entirely 

 embedded in original clastic grains of quartz. 



Besides the above minerals of usual occurrence, small quartz grains of 

 different orientation from the matrix are very rarely found included in the 

 large qu.artzes of the general background. Only two or three such cases 

 have been observed, and in these the included grain is surrounded almost 

 wholly with thin plates of mica. It is believed that these are original 

 clastic grains which, perhaps because protected by a film of material now 

 represented by the micas, have escaped the general fate of their neighbors. 



One or two composite inclusions, made up of microcline, the micas, and 

 quartz, have also been noticed. These seem to represent original pebbles 

 of granite or a crystalline schist. 



The pressure effects begin with the appearance of optical strain and 

 decided elongation in the large quartzes of the groundmass. This is fol- 

 lowed by fracture, either along or quite independent of the original sutures, 

 the crack often halting in the interior of a grain. The fractures preserve 

 very roughly the same general direction, but frequently intersect at very 

 acute angles, or come together in sweeping curves. The breaking is fol- 

 loAved by movement, and this results in the production of a fine-grained 

 quartz mosaic between the parted surfaces. In the final stages shown in 

 the series of slides in my collection, the rock is made up of long, narrow 

 lenses, each of which is an enormously strained quartz individual, separated 

 by narrow anastomozing zones of very finely subdivided quartz. After the 

 fracturing took' place there seems to have been no further distortion of the 

 lenses, for the edges of adjacent individuals follow similar curves, which are 

 often reversed, and in many cases could be brought together with an 

 accurate fit. 



If the Sturgeon quartzite represents an original sandstone, it is evident 

 from the facts stated above that the old quartz grains have undergone com- 

 plete recrystallization. The usual conception, since the time of Sorby, of 

 the process by which quartzites are formed from original deposits of sands 

 is that new quartz is deposited around eacli original fragmental quartz grain, 



