628 ME. E. GEEENLY ON THE GEOLOGY OF THE [Nov. 1 896, 



The pisolitic ironstone itself also contains a few lath-shaped 

 felspars. Though gradations exist, it does not pass imperceptibly 

 into the tuffs, but occurs in a well-defined bed about 18 feet thick. 



The section is a little obscured by small faulting, but there appear 

 to be tuffs above and below. The whole series is about 50 feet 

 thick, and probably lies in a gentle synclinal fold. 



It is clear, therefore, that we have here a record of a volcanic 

 episode contemporaneous with the latter part of the great Arenig 

 eruptions of Merionethshire. And it would seem to be due to a 

 separate outburst in this region, and not merely wind-borne from 

 the south, otherwise the pisolite and shale could hardly be so free 

 as they are from volcanic material. 



(I may add that, in the Didy?nograptus-beds at Caernarvon, I 

 found beds of very similar tuff, lying as hard ribs among the dark 

 shales at Pont Seiont. I have failed to find any, however, in the 

 shore-section east of Port Penrhyn at Bangor.) 



There is no perceptible cleavage, but there has been powerful 

 earth -movement, the shales at Careg Onen folding over, and 

 plunging against the schists of the western end of the section. 



Llanddona. — A tract of Ordovician shales must exist, almost 

 entirely concealed by Boulder Clay, skirting the coast for about 

 | mile on either side of Llanddona Church, and extending about 

 ^ mile inland. 



While mapping the drift-covered slopes I observed a great number 

 of blocks of black mudstone, along with which were also many of 

 pisolitic ironstone, and the drift itself was full of small angular 

 fragments of black shale. At last two small exposures of shale, 

 like that of Careg Onen, were found at Corn-ud. They dip W.S.W. 

 at 25°, and are probably faulted against the schists. 



I think it is safe to assume that this area is occupied by such 

 shales (its featuring, moreover, is quite unlike that of the schists), 

 and have therefore inserted it on the maps. 



One or two of the boulders have yielded graptolites — * Climaco- 

 grajrtus, of a type that occurs in the Llandeilo-Caradoc rocks,' 

 Mr. Peach writes, adding that he ' should think they belong to 

 C. bicornis, but, without seeing their proximal ends, would not like 

 to say anything further/ 



V. The Cakbonifeeotts Rocks. 



These rocks are noticed briefly in Ramsay's ' Geology of North 

 Wales,' pp. 258-59. They present some points of general interest. 



The prevailing type is a light grey, rather crystalline limestone ; 

 but there are many interesting varieties, particularly a thick mass 

 of brown dolomite with pyrites and other heavy minerals. 



Most striking, however, are the sandstones and conglomerates 

 which occur near the East Point, and at Seiriol and Fedw Fawr, 

 near the middle of the area. They are about 90 or 100 feet thick, 

 and consist chiefly of very fine white sandstone, with pyrites and 

 carbonaceous particles, and partings of soft grey shale. Obscure 



