Geological Society. 153 



the new striae from one another and from the old ones are the same 

 as those of the old ones from one another. 



IX. The principal influence of a change in the electromotive 

 force appears to consist in altering the velocity of proper motion. 

 A change in the amount of battery-surface exposed produces 

 a corresponding change in the duration of the entire discharge, 

 as well as, apparently, in the development of some of the minor 

 details of the striae. 



X. When the proper motion of the elementary striae exceeds a 

 certain amount, the striae appear to the eye to be blended into one 

 solid column of light, and all trace of stratification is lost. When 

 this is the case the mirror will often disentangle the individual 

 striae. But there are, as might well be expected, cases in which 

 even the mirror is of no avail, but in which we may still suppose 

 that stratification exists. A variety of experiments have led me 

 to think that the separation of the discharge into two parts, viz. 

 the column of light extending from the positive terminal, and 

 the glow around the negative, with a dark space intervening, may 

 be a test of stratified discharge ; but I cannot affirm any thing 

 certainly on this point. 



GEOLOGICAL SOCIETY. 



[Continued from p. 7o.~] 



December 20th, 1876.— Prof. P. Martin Duncan, M.B., F.R.S., 



President, in the Chair. 

 The following commmunications were read : — ■ 



1. " On Pharetrospongia Strahani, a fossil Holorhaphidote Sponge 

 from the Cambridge Coprolite Bed." By W. J.Sollas,Esq.,B.A.,F.G.S. 



2. " On the Remains of a large Crustacean, probably indicative of 

 a new species of Eurypterus, or allied genus {Eurypterus ? Stevensoni), 

 from the Lower Carboniferous series (Cement-stone group) of Ber- 

 wickshire." By Bobert Etheridge, jun., Esq., F.G.S., Palaeontologist 

 to the Geological Survey of Scotland. 



3. " On the Silurian Grits near Corwen, North Wales." By 

 Prof. T. McKenny Hughes, M.A., E.G.S. 



The author commenced with a description of sections near 

 Corwen in North Wales, from which he made out that the grits 

 close to Corwen were not the Denbigh grits, but a lower variable 

 series, passing in places into conglomerate and sandstone with 

 subordinate limestone and shale. This series, under the name of 

 ' The Corwen Beds,' he described in detail, having traced them round 

 the hills S. of Corwen, also near Bryngorlan, S. of the Yale of 

 Clwyd, on Cyrnybrain, and S. of Llangollen. He had noticed in 

 places a kind of double cleavage affecting the lower series but not 

 the upper, and also fragments of cleaved mudstone included in the 

 upper, from which he inferred a disturbance of the older rocks 

 previous to the deposition of the newer. He exhibited a selection of 

 fossils, and said that immediately below the Corwen beds there were 

 none but Bala fossils. In the Corwen beds all the few fossils found 

 were common to the Llandoverv rocks, some of them, as Meristella 



