in America and Europe. 281 
would be easy to extend further this review of late advances 
made in the study of the ancient crystalline rocks, but the 
writer has preferred to confine himself to those regions which 
he has lately examined. 
CoNCLUSIONS. 
1. The Pebidian of Hicks has both the lithological charac- 
ters and the stratigraphical position of the Huronian of North 
America, to which he has already referred it. 
2. The Arvonian is, in like manner, identical with the 
Hailleflinta group of Sweden ‘and with the Petrosilex group of 
orth America, which I had provisionally included in the 
lower part of the Huronian, and which Hitchcock subsequently 
called Lower Huronian. The fact that there is in Wales a 
Sstratigraphical break between it and the overlying Huronian, 
will help to explain the frequent absence of the Arvonian at 
the base of Huronian in many of its American localities. 
The Dimetian, including the granitoid and gneissic 
rocks with limestone ‘bands, so far as can beseen in the limited 
North t was from a misconception that Dr. Hicks 
in 1878 provisionally ieheedd the Dimetian to the Upper Lau- 
rentian, a name ne time use by the geological survey of 
Rosine fs from and older than the Dimetian. These two appar- 
ently correspond to the Ottawa and Grenville divisions of the 
Neca aeseeitine 3 in Canada, and perhaps to the Bojian and 
ercynian gneisses of Giimbel, in Bavaria. 
[The © following is I partial list of publications relating to the rocks noticed in 
aid re of this 
© Quar. Jour. ‘Geol. Soe. of London are the following papers on these 
in Wales: Hicks, May, 1877. p. 230; Hicks & Davies, Feb., 1878, p. 147, ae 
May 1878, p. 153; Hughes & Bonney, Feb., 1878, p. 137; s & Davies, May, 
1879, p. 285: Hicks & Bonney, ibid., p. 295; Bonney, ibid 309 nese 
Houghton, ibid., p Hughes, Nov., 1879, p : Maw, Aug., 1878, p. 764; 
icks, rock -shire, , 1878, - 811 ppse _ Rocks of St. 
Vi roc. Bristol Naturalists’ Society, v a 2, 
1. ii, 
On these rocks in Shropshire, in the same | Sauna! : ‘Aloe 2 877, 
rely fa sie 1877, p. 653, and Aug., 1878, p. 754; Callaway te jv Pay 
een n Charnwood Forest, in the same journal, Hill and Bonney, 
ee 1877, p. 753, et May, 1878, p. 199. 
farther Hunt, Chemical and Geological Essays, pp. 34, 269, 270, 272, 278, 
pond also his Azoic Rocks, part i (Second Geol. Survey of Penn., 1878), pp. 187, 
For the rocks of the Ardennes see Memoire sur les Roches dites Plutoniques, 
ete. (4to, pp. 264), by de la Vallée Poussin and Renard, from Memoires de la 
