PRE-CAMBRIAN CLASSIFICATION IN ONTARIO 591 



The intrusive granite occupies a considerable area on the coast of Lake 

 Huron, south of Lake Pakokagaming. It there breaks through and disturbs 

 the gneiss of the Laurentian series and forms a nucleus from which emanates 

 a complexity of dykes, proceeding to considerable distances. As dykes of a 

 similar character are met with intersecting the rocks of the Huronian series 

 [Timiskamian], the nucleus in question is supposed to be of the Huronian age, 

 as well as the greenstone dykes which are intersected by it/ 



If Logan mistakenly applied the name Laurentian to his 

 "Huronian granite" in certain areas, why should his mistake be 

 perpetuated? Now that we know these granites and gneisses 

 in some localities belong to what he called Huronian granite, why 

 should we apply the name Laurentian to them instead of to the 

 group that is much older, the Laurentian of our classification and 

 the "Granites Eruptive through the Keewatin" of Coleman's? 



There are certain objections to giving the names of systems or 

 series to intrusives such as the Laurentian and Algoman. For 

 this reason the authors prefer to inclose the names in parentheses 

 in the table of classification, thus: (Laurentian intrusives) and 

 (Algoman intrusives), or (Laurentian) and (Algoman). These 

 intrusives occur in such great volume in the pre-Cambrian that it 

 would seem well to retain distinctive age names for them. Age 

 names are especially useful in economic work, our most important 

 gold deposits, for example, being genetically connected with the 

 Algoman. 



DUAL SUBDIVISION 



A dual subdivision of the pre-Cambrian, as shown in Table II, 

 is employed by many authors. It is purely arbitrary, and is now 

 known to be based on a misconception as to the. relations of the 

 rocks. It was believed, or assumed, that the greatest uncon- 

 formity within the pre-Cambrian is that at the base of the "Lower 

 Huronian," as the term is used by the U.S. Geological Survey. 

 Such, however, has not proved to be the case. There is no proof 

 that the unconformity at the base of the Timiskamian is of less 

 magnitude than that at the base of the Animikean, of the authors' 

 classification, or vice versa. According to Coleman, the various 

 sedimentary series of the Timiskamian, his Sudbury series, where 



' Geology of Canada, 1863, p. 58. 



