50 



PURCELL SILLS. 



The Purcell sills are not only of scientific interest 

 but, economically, they contain small deposits of copper 

 ores. The sills occur as sheets of igneous material from 

 6 to 2,000 feet (i-8 m. — 609 m.) in thickness, intruded 

 between the bedding planes of the quartzites, and occa- 

 sionally as very small pipes about 400 feet (121 -6 m.) 

 in diameter. Most of these sills are composed of gabbro, 

 but a few show great variations within the same magmatic 

 chamber. The same sill, although believed to be simple 

 in character, is heterogeneous in composition, that is 

 to say processes of differentiation have evidently affected 

 the magma of these sills before solidification with the 

 result that the material of some of the sills is stratified 

 according to density. In this case, a granite layer appears 

 at or near the upper contact of the sills, passing downwards 

 into gabbro. The thickness of the granitic layer bears 

 no relation to the thickness of the sill. The sills have 

 probably been affected by all the movements which the 

 enclosing sediments have undergone, and hence occur 

 in all attitudes, from horizontal to vertical. The sills 

 have evidently reached their present stratigraphic position 

 through fissures, although very few dykes have been found 

 within the region examined. The age of these intrusives 

 is probably Cambrian. 



GATEWAY FORMATION. 



The lower part of the formation consists of alternating 

 bands of massive concretionary siliceous dolomite and 

 limestone, weathering buff, and massive light grey 

 quartzites. These are succeeded by thin bedded, sandy, 

 argillites and greenish-grey, siliceous, argillites. The 

 sandy argillites weather a light buff and are characterized 

 by the presence of abundant casts of salt crystals. 



PHILLIPS FORMATION. 



The Gateway passes gradually into the overlying 

 Phillips formation which consists of dark, purplish, and 

 red metargillites, and sandstones with thin laminae of 

 greenish siliceous argillite intercalated at several horizons. 



