LOCATION OF THE GLASS-BRECCIA. 139 



ship it forms a sharp elbow apparently getting around an anticlinal axis and runs off 

 to the eastward on the southern side of Vermilion lake ; but here, as above stated, it 

 passes into a slaty conglomerate holding pebbles of syenite. In this form it is trace- 

 able about ten miles more. 



"A good section of the typical form of the breccia may be seen in the cuttings near 

 Onaping, where the Canadian Pacific railway intersects it, twenty-three miles north- 

 west of Sudbury Junction. It has an average breadth of fully a mile, and as it dips at 

 angles of 45° and upwards it must have a thickness of over 4,000 feet. Owing to its 

 hardness and toughness it has resisted denudation better than the sandstones and 

 argillites, and it rises a few hundred feet above the latter in the form of a range of 

 rugged hills overlooking the comparatively level country on the southeast. Along its 

 northwestern side it is separated, in places at least, from the quartz-syenite by a 

 massive band of ash-gray quartzite containing usually an abundance of white quartz 

 pebbles scattered through it. 



" It was supposed that from its compact nature this breccia might be used in orna- 

 mental construction ; but, while it gives a good, smooth surface, it has not been found 

 susceptible of fine polish." 



In a hand specimen this rock presents a nearly black felsitic matrix, in 

 which are embedded sharply angular or slightly rounded fragments, vary- 

 ing from li cm. in diameter downwards to ultra-microscopic dimensions. 

 These fragments are lighter in color than the matrix, but differ considerably 

 among themselves in their tint, structure and composition. The majority 

 resemble chalcedony in appearance, others are greenish, while some of the 

 largest fragments are now replaced by a single calcite individual. Occa- 

 sional small grains of clear vitreous quartz may also be detected, while specks 

 of magnetic pyrites (pyrrhotite) are everywhere abundant. Many of the 

 angular fragments show distinctly under the lens a flow or vesicular struc- 

 ture, which is still more apparent in a thin section of the rock when seen 

 under the microscope. 



Figure 4.— Section of silicified Glass-Breccia. 



The appearance of this rock when viewed with a low magnifying power 

 (X 20 diameters) is shown in the accompanying figure 4, for which I am 



