400 CARBONIFEROUS LIMESTONE NEAR LLANDUDNO. [Aug. 1 898, 



in the areas referred to, been converted into dolomite appears 

 hitherto to have been overlooked. 



Mr. Strahan^ remarked on the excellence of the field-work upon 

 which this paper was founded. The dolomite had been compared 

 by the Author with a Permian rock : it differed, however, in being 

 an alteration-product. The alteration in this and other cases 

 affected the limestone most capriciously, converting it into an 

 aggregate of crystals and completely obliterating the organic 

 structures. Moreover it frequently accomj)anied faults, as in the 

 case of the 'dun limestones' of the North of England. The 

 percentage of magnesium carbonate had been stated by the Author 

 as 48, but this seemed considerably too high for an ordinary dolomite. 

 It might have been thought that the Trias, with the Dolomitic 

 Conglomerate at its base, had been the source of the magnesia ; but, 

 as a matter of fact, the usual alteration effected by the Eed Eocks 

 had been the conversion of the limestone into haematite. Moreover, 

 the Dolomitic Conglomerate was often almost devoid of magnesium 

 carbonate. On the other hand, the dolomitization frequently 

 accompanied faults which may, at some former period, have con- 

 ducted underground water from the Coal Measures into the limestone. 

 It was interesting to note that the rock found by the Author to 

 lie upon the limestone differed from the purple sandstones which 

 occupied that position in the Yale of Clwyd, as well, of course, as 

 from the chert-beds of Flintshire. He congratulated the Author 

 on his excellent results. 



Mr. Marr also spoke. 



The Author, in reply, stated that the subjects brought forward 

 by the speakers would take up too much time for comment, and 

 unfortunately he had not been able to hear all that had been said. 

 He was much pleased with the interest taken in the paper, for he 

 had not been able to read the whole of it, and some details had of 

 necessity been omitted. 



