Correspondence — /. Wilfrid Jackson. 479 



IX. — Fkozen Muck in the Klondike District, Yukon TEEKiTORr, 

 Canada. Ey J. B. Tyerell, E.R.S.C. Trans. Eoy. Soc. 

 Canada, ser. iii, vol. ix, pp. 39-46, with 3 plates, 1917. 

 riIHE valley floors of the Klondike District are the products of the 

 X third cycle of erosion since the last continental uplift of the 

 region. In the Miocene period the Dome peneplain was produced by 

 the first cycle of erosion. In Pliocene times the valleys in which 

 the older White Channel gravels were deposited were carved out 

 during the second cycle of erosion, while the present valleys and 

 their alluvial gravels are connected with the third cycle, which lasted 

 till tlie end of the Pliocene period. During this time the climate 

 was temperate and the country was inhabited by a number of the 

 larger mammals, but at the beginning of the Glacial period different 

 conditions set in, and, though this region was not covered by an 

 ice-sheet, the soil was certainly frozen all the year round. In con- 

 sequence of this the alluvial gravels and the beds of the streams 

 became impervious to water and resistant to erosion. When, 

 therefore, the snow melted in the spring the water in the stream 

 channels brought down, instead of sand and gravel, only vegetable 

 debris from the hill-sides, which collected on the alluvial flats and 

 was held fast and preserved by the large growth of bog mosses. 

 In this way great thicknesses of this frozen bog or "muck" were 

 accumulated, varying from 2 to 40 feet and even 100 feet in the 

 narrower gulches, which have to be sunk through before the gold- 

 bearing gravels can be worked. " Muck " is also found in the form 

 of frozen bogs on the hill-sides, where it often contains layers of 

 clear ice,^ tilted at steep angles by the slipping of the bog. The 

 "muck" now forms the upper part of the valley deposits, which 

 shows that little or no gravel has been transported since the 

 beginning of the period of perennial frost, and that, therefore, the 

 valley gravels are all pre-Glacial in age. 



COE, I?. ESI^Olsr ID EllsrCE. 



ON TEREBBATULA GRAYI, DAVIDSON. 



Sm, — In a former paper in this Magazine (Dec. VI, Vol. Ill, 

 pp. 21-6, 1916) I proposed the name Thomsonia for the Terehratula 

 grayi of Davidson. This name. I find, has unfortunately been used 

 for Insecta on two previous occasions, viz. in 1879 and 1884, and, 

 therefore, cannot stand. In its place I now propose 

 CoPTOTHTEis, gen. nov. 



Coptothyris grayi has been placed in Waldheimia (now Magellania) 

 and in Dallina by various authors, on account of the loop having 

 reached the highest developmental stage in the Terebratellidse ; but 

 it is distinct from either of these genera on other grounds. The full 

 details of these differences are reserved for a future paper on the 

 cardinalia of the Dallininse in general. In this paper I hope to 

 show that the cardinalia (or hinge-processes of the dorsal valve) of 

 the sub-family Dallininse can be readily diff'erentiated into, at least, 



