15 

 THE GEOLOGY OF SUDBURY. 



The town of Sudbury rests mainly on stratified clay under- 

 lain by quicksand formed in glacial Lake Algonquin, but 

 hills of rock project above these lake beds, showing surfaces 

 that have been smoothed and striated by Pleistocene gla- 

 ciers. The chief rock within the town is McKim gray- 

 wacke, which is well stratified with thin slaty layers the 

 bedding showing distinctly on weathered surfaces. The beds 

 are usually steeply tilted and are even vertical against a lac- 

 colithic mass of gabbro toward the east side of the town. 

 The strike and dip vary considerably and in many places 

 the greywacke is brecciated and recemented, the crushing 

 having taken place, it is supposed, during the advent of the 

 nickel eruptive. The graywacke is often crowded with 

 small pseudomorphs after staurolite, suggesting contact 

 metamorphism, which may be accounted for by the effects 

 of the laccolithic gabbro and other eruptives in the region. 



Toward the southeast on the shores and islands of Ram- 

 say lake, the graywacke is followed by pale gray quartzite, 

 well stratified in thick layers which are often cross bedded. 

 They have usually a dip of about 45 deg. with a strike of 

 northeast and southwest. The Ramsay lake quartzites form 

 an extensive group of rocks, having a width of six miles, 

 where widest, and an estimated thickness of 15,000 feet. 

 They appear to overlie the graywacke, though well exposed 

 contacts have not been found. 



Areas of greenstone of greatly weathered gabbro pene- 

 trate the quartzite in various places, and granite or granitoid 

 gneiss of a Laurentian type cuts them toward the south and 

 southeast. 



The quartzite, graywacke and a recrystallized arkose, ris- 

 ing as ridges somewhat to the west of the town, make up 

 the Sudbury series, which is not less than 20,000 feet thick. 



The most interesting of the eruptives penetrating the 

 rocks just described forms a laccolithic range of hills in the 

 eastern part of Sudbury, where gabbro has tipped up beds 

 of graywacke and sometimes even overturned them slightly. 

 The gabbro is gray-green and much weathered, consisting 

 now mainly of hornblende and poorly preserved plagioclase; 

 but at various points on the hills there are large patches of 

 white rock, either " roof pendants " of quartzite partly di- 

 gested, or segregations of a pegmatitic kind. These patches 



