190 Correspondence—Mr. William Swanston. 
show that most of the pedestals of these boulders must have existed 
before the arrival of the boulders, while the pedestals acquired 
through the boulders protecting the underlying rock from denudation, 
were generally imperfectly formed. On the Clapham plateau he 
found that the average vertical extent of denudation around the 
boulders with acquired pedestals was not more than on the Eglwyseg 
plateau, or about six inches. In the case of boulders which are not 
well adapted to concentrate rain-water, the extent of lowering of 
the surrounding rock-surface was often inappreciable; and this ac- 
counted for the continuous extension of flat limestone rock-surfaces 
under some of the boulders. The author then described what he 
had found to be preglacial as well as postglacial rain-grooves on 
limestone rock-surfaces, near Minera and on Halkin mountain (North 
Wales), where he found the average depth of those of the grooves 
which were probably postglacial to be about six inches. In con- 
clusion the author entered into a consideration of the time which 
has elapsed since the close of the glacial period, and stated the main 
results of his observations as follows :— 
1. That the average vertical extent of the denudation of limestone 
rocks around boulders has not been more than six inches. 
2. That the average rate of the denudation has not been less than 
one inch in a thousand years. 
3. That a period of not more than six thousand years has elapsed 
since the boulders were left in their present positions by land-ice, 
floating ice, or both. 
3. * Notes on the Corals and Bryozoans of the Wenlock Shales 
(Mr. Maw’s Washings).” By G. R. Vine, Esq. Communicated by 
Prof. P. Martin Duncan, M.B., F.R.S., V.P.G.S. 
The author briefly discussed the views of different writers upon 
the systematic position of the genera Cheietes, Monticulipora and 
their allies, and also of the forms referred to the Polyzoa, and gave 
a list of 89 species and varieties of Corals and Polyzoa obtained by 
him from Mr. Maw’s washings of deposits belonging to the Wenlock 
series in Shropshire. These forms were referred by him to the 
genera Dekayia, Monticulipora, Callopora, Heliolites, Thecia, Favo- 
sites, Syringopora, Halysites, Coenites, Cyathophyllum, Lindstreemia, 
Cladopora, Leioclema, Ceriopora, and Ceramopora. New species 
are Leioclema granatum and pulchellum. 
CO SON DEC 
a 
MR. HOWORTH ON IRISH GLACIAL DRIFTS. 
Sir,—Some errors have crept into Mr. Howorth’s reference to the 
Trish Glacial Drifts, in your February Number. Bovevagh (not 
Boreragh) in Co. Derry, where the fossiliferous glacial deposits in 
the North of Ireland attain their greatest altitude, is 450, not 1150 
feet above the sea-level, and this particular bed does not yield either 
Nucula oblonga (Leda pernula) or Cyprina, as stated on authority of 
Mr. W. A. Bell (Guou. Maa. Vol. X.). Portlock only mentions 
