224 A. P. COLEMAN—CLASTIC HURONIAN ROCKS. 
facts employed have been observed by the writer while engaged in field 
work for the Ontario Bureau of Mines. 
CLASSIFICATION OF THE Rock SERIES 
Lawson describes the region as wholly Archean, and divides the rocks 
into two parts—a lower one, the Laurentian, and an upper one, the Onta- 
rian, further subdivided into the Couchiching and the Keewatin. The 
term Laurentian is used here in a petrographic sense, to denote a series 
of entirely crystalline, generally acid, rocks, especially coarse grained 
gneisses and granites, in spite of the fact that these rocks have an eruptive 
contact with the overlying series, and so are later in age than the Ontarian. 
The Couchiching rocks, found chiefly in the southern portion of the 
region along the boundary of Minnesota, consist in general of monot- 
onous gray or brown mica-schists and gneisses, sedimentary in origin. 
The Keewatin, which overlies the Couchiching where the latter is 
present and in other cases rests on the Laurentian, is largely composed 
of eruptives and their products—schists resulting from shearing, ash 
rocks, and agglomerates—basic in the lower part, acid in the upper.* 
Lawson does not definitely correlate the Couchiching with either the © 
Laurentian or Huronian, but Van Hise includes at least part of it in the 
Basement Complex.t Lawson puts the Keewatin doubtfully with the 
Huronian, pointing out numerous differences between it and the original 
Huronian as described by Logan. In general, however, Canadian geol- 
ogists do not hesitate to class these altered eruptives and the accompany- 
ing sedimentary rocks as Huronian, and this usage will be followed in the 
present paper. : 
In the first place the Huronian clastics will be briefly described, and 
afterwards the relations subsisting between the series of rocks and their 
causes will be discussed. The term Archean will be used to include both 
Laurentian and Huronian (or Ontarian), though according to the classi- 
fication adopted by the United States Geological Survey the latter would 
be included in the Algonkian. 
HuRoNIAN CLASTICS 
GENERAL STATEMENT AS TO ORIGIN 
As to origin, the Archean clastic rocks of western Ontario are of three 
kinds—pyroclastics, autoclastics, and sedimentary rocks proper. 
The pyroclastics, consisting of agglomerates, ash rocks, etcetera, are of 
* Geol. Survey of Canada, 1887, p. 22, F, ete. 
+ Principles of pre-Cambrian Geology, p. 782. 
