44 



TWELFTH REPORT. 



is a sharp division liuc. Tlio oompositiou of one stratum is often 

 fairly constant for some distance ; but in some cases a distinct cliange 

 takes i)lace in a few feet laterally as well as vertically. 



ROCKS OF THE NIPISSING SILVER FIELDS. 



f Recent Clay, marl. peat. 



I. Cenozoic. . . -I Pleistocene (1). Coar.-^e unstratitied material — .sand, gravel, bowlders; (2). 



L Stratitied clay with .«ome .sand. 



Great unconformity: 

 [ Silurian. 

 II. Palaeozoic... ^ 



Great unconformitii: 



' Keweenawan . 



III. .Vlgonkiaii . 



C.rey limestone with some interbedded greenish shales, and at 



the ba.se an arenaceous conglomerate. 

 Correlated with Niagara of New York State. 



• Igneous intrusives only. Chiefly quartz diabase and quartz 

 gabbro with acid differentiation proaucts. Some olivine diabase 

 and diabase p;>rphyritc dykes. 



Igneous contact: 

 Htironian Sedimentary rocks only. 



(a) .\n upper series. Probably equivalent to Middle Huronian 

 of Lake Superior district. Chiefly feldspathic quartzite with 

 some conglomerate. 



Sligfit unconfonniti/: 



(b) A lower series. Probably equivalent to Lower Hurotiian of 

 Lake Superior district. Chietly greywacke. shale, conglomerate, 

 and feldspathic quartzite. The ' conglomerate pebbles are 

 mostly of holocrystalline igneous rocks, the matrix greywacke 

 and grey shale. The rocks are seldom schistose except as the 

 result of contact metamorphism. 



Great unconformitii: 



f Laureniian. 



IIV. .Vrchean.. 



.Igneous intrusives only. Holocrystalline light colored siliceous 

 rocks. Chiefly granites, diorite, syenites and gneisses. 



Igneous contact: 

 Keewatin. . . ■ . .Igneous and sedimentary rocks. All much metamorphosed, many 

 being chlorite, hornblende and sericite schists. 

 The relative age of the igneous and sedimentary rocks is doubtful 

 The agglomerates were probably contemporaneous with some of 

 the non-clastic volcanic rocks, and may be contemporaneous 

 with the other sediments. The igneous rocks are chiefly of 

 extrusive types. 



E-rtrusires . 



Intrusives. 



Sediments. 



. (1) Dark colored basic rocks — basalts — mostly 

 with composition and texture of altered dia- 

 bases. 

 (2) Light colored siliceous rocks — felsite por- 

 phyries — mostly quartz porphyries which have 

 been altered to" sericite schists. 



. (1) Basic rocks, mostly diabase and gabbro. 

 (2) Siliceous rocks, mostly quartz porphyries and 

 porphyrites, 



. (1) The iron formation, chert, jasplite, carbonates, 



slates and green schists. 

 (2) Fragmental volcanic rocks — a grey felsite 

 agglomerate. 



The shales are for the most part of j>rey color, less often greenish 

 black. Occasionally they are interbanded with layers of purple, green, 

 and pale gray colors. The ihief recognizable minerals are quartz and 

 altered feldsi)ars, minnte scales of chlorite and sericite, and small grains 

 of epidote, titanite and iron ores. In mineralogical and chemical com- 

 position they are not nullke green shales of other formations. 



The qnartzites are in most instances feldspathic and grade insensibly 

 into typical arkoses. They are usually very massive, fine to medium 

 grained and not unlike light colored granite in apjvarance. It is often 

 very diflicult to determine their strnttnre. as they are seldom well 

 marked bedding i>lanes. There are however instances in which the 



