Geological Societij of London. 43 



gradually dries up, a surface of extremely fine calcareous mud is 

 exposed. This deposit is often very finely laminated, and occasion- 

 ally among the laminse old surfaces can he discovered, which, after 

 having been exposed for some time to the air, had been covered up 

 by a fresh inflow of watery mud into the pit. The author described 

 the character of the cracks made in the process of drying, and the 

 results produced when these were filled up. He also described the 

 tracks made by various insects, indicating how these were modified 

 by the degrees of softness of the mud, and pointed out the difierences 

 in the tracks produced by insects with legs and elytra, and by 

 Annelids, such as earthworms. The marks made by various worms 

 and larv89 which burrow in the mud were also described. Marks 

 resembling those called Nereites and Mijrianites are produced by 

 a variety of animals. The groups of ice-spicules which are formed 

 during a frosty night also leave their impress on the mud. The 

 author concluded by expressing the opinion that Cruziana, Nereites, 

 Crossopodta, and Paloeocho7-da were mere tracks, not marine vegetation, 

 as has been suggested in the case of the first, or, in the second, the 

 impression of the actual body of ciliated worms. 



11. -December 5, 1883.— J. W. Hulke, Esq., F.R.S., President, in 

 the Chair. — The following communications were read : — 



1. " On the Cambrian Conglomerates resting upon and in the 

 vicinity of some Pre-Cambrian Eocks (the so-called intrusive masses) 

 in Anglesey and Caernarvonshire." Bj^ Henry Hicks, M.D., F.G.S. 



The author stated that as he and others who had asserted that 

 some of the great rock-masses marked as ''intrusive" on the maps 

 of the Geological Survey in Anglesey and Caernarvonshire were 

 really of Pre-Cambrian age, had been charged in the last edition of 

 the Geological Survey Memoir on North Wales with having done so 

 on purely theoretical grounds, he had found it necessary to obtain 

 the additional evidence which he now brought before the Society. 

 He felt this to be the more incumbent upon him as the present 

 Director-General had recently stated that his predecessor would not 

 admit the existence of any Pre-Cambrian rocks in the Principality, 

 and had further asserted that the author, " with the most complete 

 disregard of the evidence by which the officers of the Survey were 

 led to regard certain rocks as intrusive " in Cambrian and Silurian 

 strata, had simply made these rocks into metamorphic and volcanic 

 Pre-Cambrian masses, without giving any detailed statement of the 

 evidence in support of so great a change. The author said that he 

 was pi'epared to bring forward conclusive evidence of the correctness 

 of his views from all the areas referred to, and he offered the present 

 paper as a first instalment of the detailed criticism demanded by the 

 Director-General. 



In a former paper he had maintained that there was no evidence 

 to show that the so-called intrusive granite in Anglesey had altered 

 the Cambrian and Silurian rocks in its immediate vicinity, or that 

 they had been entangled in it as described, but that it seemed to be 

 a rock of metamorphic origin, varying much in its general appearance 



