212 AUTHORS' ABSTRACTS 



ioo feet deep was cut at Aqueduct, near Schenectady. Certain beds of 

 massive, water-laid clay west of Little Falls, taken with similar depos- 

 its in the Chenango and Unadilla valleys, are thought to show long 

 and quiet deposition, with perhaps considerable later erosion, before 

 the last advance of the ice across central and southern New York. 



Clastic Huronian Rocks of Western Ontario and the Relations Between 

 Laurentian and Huronian. By A. P. Coleman. 



The rocks of the Lake of the Woods and Rainy Lake regions of 

 western Ontario have been excellently mapped by A. C. Lawson, who 

 calls them Archaean and subdivides them into a lower part, the 

 Laurentian, and an upper one, the Ontarian, further subdivided into 

 the Couchiching and the Keewatin. The Laurentian, which consists 

 chiefly of granite and gneiss, underlies the other two series, but has 

 an eruptive contact with them, showing that it was the latest in age. 

 The Couchiching is formed mainly of fine-grained gray gneiss and 

 mica schist, of clastic origin, since the quartz is usually in distinctly 

 rounded grains. These rocks merge into almost unchanged sand- 

 stones in a few places, as found by the writer. 



The Keewatin is much more varied, consisting very largely of 

 basic and acid eruptives with their pyroclastics ; but containing also 

 important sedimentary members, such as limestone, slate, quartzite 

 and conglomerate. The last rock is not a basal conglomerate resting 

 on the Couchiching or Laurentian, but comes high up in the series, 

 since it contains mainly pebbles of eruptives and schists found in 

 adjoining portions of the Keewatin. It may represent a break equiva- 

 lent to that between the lower and upper Huronian in the states to the 

 south, as described by Van Hise. These conglomerates contain no 

 Laurentian pebbles so far as known. 



The field relations of the three formations are very interesting, 

 both as mapped by Lawson and as observed on the bare shores of 

 lakes in the western Archaean peneplain. The Keewatin, and in the 

 southern part of the region the underlying Couchiching, form sharp 

 synclines, curving as wide meshes round the areas of Laurentian, 

 which vary greatly in size, running from a diameter of less than a mile 

 to about fifty miles. Starting from the center of a Laurentian area 

 one commonly finds first granite, then gneiss having a strike parallel 

 to that of the adjoining schist. Before reaching the schist many frag- 



