480 Geological Society. 



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

 fields, Ireland, Edinburgh, Arran, &c. 



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 

 in 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 refer- 

 ence to a group of very large and far-travelled erratics in Llanarmon 

 parish, Denbighshire." By D. Mackintosh, Esq., F.G.S. 



The author, after referring to a number of northern- drift boulders 

 in addition to those he had noticed in a former paper, describes 

 several large felspathic boulders found up to a height of about 

 1750 feet above the sea, on Cefn-y-fedw, N. of Llangollen. He 

 then gives a somewhat detailed account of the drifts in the neigh- 

 bourhood of Corwen, and of some large felspathic boulders, probably 

 from the Arenig mountains, which are generally found on the sur- 

 face, or interposed between the lower boulder-clay and an upland 

 extension of the middle sand of the plains. The main part of the 

 paper is devoted to an account of the discovery of a numerous group 

 of very large and far-travelled felspathic boulders in the parish of 

 Llanarmon, Denbighshire. The author refers particularly to a remark- 

 able slickensided boulder, and to the great " Immovable Stone " at 

 Maendigychwyn (now called Eryrys), about 1150 feet above the sea, 

 which is the largest far-transported boulder he has heard of in the 

 British Isles. He stated a number of facts and considerations 

 which led him to believe that the Llanarmon boulders, along with 

 those further N. and W., must have come all the way from Snow- 

 don, and that they were floated over passes or cols in the intervening 

 ranges of hills by icebergs or coast-ice about the close of the Lower 

 Boulder-clay period. He concluded by noticing the necessity for a 

 personal examination of boulders instead of relying on answers to 

 queries, and stated that about Llanarmon the felspathic boulders 

 are called " granite tumblers," while in Cheshire all kinds of boulders 

 are called " marble stones." 



6. " Note on the Bingera Diamond-fields." By Archibald Liver- 

 sidge, Esq., F.G.S. 



The author commenced by describing the general characters of 

 the older Australian Diamond-field of the Mudgee or Cudgegong 

 District. The Bingera Diamond-field is situated in a basin among 

 the mountains of the Drummond range, the encircling hills being of 

 Carboniferous or Devonian age. The diamantiferous drift occurs 

 in patches in the basin, which is invaded by spurs of basalt. The 



