424 Reports and Proceedings — 



immediate neiglabourhood. This is accounted for by the author on. 

 the supposition that they formed a series of shallow lakes, frozen 

 over in winter, and that the ice on breaking up carried away frag- 

 ments of the rocks. The distribution of the boulders on the plain 

 north of the ridges was also attributed to floating ice. 



The generally accepted marine origin of the great plain was said 

 to be negatived by the absence of sea shells in its deposits, whilst 

 Cyrena flummalis occurs in them. The author regards them as 

 deposits from a great expanse of fresh water kept back by a barrier 

 of polar ice descending far towards the south. In its greatest ex- 

 tension this ice-barrier would produce the crushing of the bed-rock; 

 and as it retreated, the water coming down from the higher ground 

 in the south would cover a continually increasing surface. 



Discussion. — Mr. Drew thought that the deposits had a lacustrine aspect. Their 

 nearest parallel was to be found in the great plain of India, the oiigin of which was 

 not quite clear, although it had perhaps originated as a delta. He did not think 

 that the ground had been covered by an ice-cap. 



Mr. Hulke believed that the author's view as to the origin of the deposits was 

 correct. He thought that the plains of Hungary were of a similar nature, their 

 freshwater deposits having been formed in consequence of the stoppage of the 

 Danube, changing what had been a dry plain into a vast lake. The effects described 

 by the author might have been produced by a rise of land in the north. 



Prof. Ansted remarked that the extent of the country under consideration might 

 be estimated roughly at 3,000,000 square miles, and asked whether this was to be 

 regarded as the area of a freshwater lake. If so, whence did the water come and 

 where did it go ? He thought such a phenomenon of very unlikely occurrence. 



Mr. Belt, in reply, said that he was glad to find that the marine theory had had 

 no supporters. He maintained as a fact that if a barrier were placed across the 

 country, the numerous rivers would soon flood its whole surface. 



4, " On the Microscopic Structure and Composition of British 

 Carboniferous Dolerites." By S. Allport, Esq., F G.S. 



The object of this paper is to supply further and conclusive 

 evidence to show that there are dolerites and basalts of Carboni- 

 ferous age whose original mineral constitution is precisely the same 

 as those of the later Tertiary periods, those of both ages present- 

 ing the same varieties of structure, and that the great alterations 

 which most of the older rocks have undergone, constitute the only 

 difference between the two groups. The author describes at some 

 length the various constituents under the following heads, viz. 

 felspar, augite, olivine, magnetite, mica, apatite, glassy matrix, etc. 

 He next describes the occurrence of dolerites in the Midland Coal- 

 fields, Ireland, Edinburgh, Arran, etc. 



In conclusion he draws attention to the many variations in com- 

 position and texture in the same rock-mass, and accounts for them 

 thus : — If the lava were simply in a viscid state, with the ingre- 

 dients imperfectly mixed, portions of it must, on consolidation, 

 contain them in various proportions, just as is known to be the case 

 it. imperfectly fused slags. 



He maintains that there is an absolute identity of composition, 

 structure, and mode of occurrence in these eruptive rocks of very 

 widely separated geological periods, and that therefore they should 

 be placed in one group. 



5. " Additional Remarks on Boulders, with a particular reference 



