G8 report — 1865. 



On an Exfemive Distribution of White Sands and Clays in North Wales 



antecedent to the Boulder Clay Drift. Bu Geohge Maw, F.S.A., F.6.S. 



if F.L.S., Vice-President of the Severn Valley Field Club. 



There exists in the counties of Carnarvon, Denbigh, and Flint, a widely distri- 

 buted series of uufossiliferous deposits older than the Boulder-clay on the evidence 

 of superposition, and differing from it in lithological character, but closety resem- 

 bling some of the Tertiary deposits in the south of England. 



They consist for the most part of white, black, and yellow sands, interstrati- 

 fied with tough, white aud grey pipeclays and earthy lignite, generally occurring in 

 pockets or egg-cup shaped depressions in the mountain limestone at altitudes of 

 from 800 to 1000 feet above the sea, but not exclusively confined to the limestone. 



Among the localities are Gwydfyd, on the Great Ormes Head, Nant y Gamer at 

 the back of the Little Ormes Head, several places on Halkiu Mountains, near 

 Holywell, at Pant du and other localities in the neighbourhood of Mold in Flint- 

 shire, — all on the mountain limestone. Also a deposit of similar white sand and 

 clay rests on Lower Silurian rocks at the back of Conway mountain, Carnarvonshire. 

 The strata are generally broken and dislocated, with a frequent tendency to a ver- 

 tical arrangement, or a concave bending of the beds into the form of the cavities in 

 which they are contained. 



A theory is referred to which has been advanced by Mr. Prestwich and others 

 to account for analogous phenomena exhibited in the excavation of sand-pipes and 

 pockets in various calcareous formations as applicable to tbe formation of the 

 Welsh deposits, viz., that the cavities were formed subsequently to the beds that 

 now occupy them, and that, as the pockets were slowly excavated by watery dis- 

 solution, the preexisting superincumbent beds were gradually lowered into them. 



Tufa deposits frequently accompany sand-pipes in calcareous formations, and are 

 supposed to result from the redeposition of calcareous matter dissolved away in 

 their excavation. 



In the valley below Caerwys a great deposit of about 200 acres of tufa exists, rest- 

 ing on boulder-clay drift, and as it was adjacent to a cavernous opening in the lime- 

 stone escarpment, it is probably connected with the subterranean dissolution of the 

 limestone and of the excavation of the pockets in the .adjacent mountain limestone 

 range. The order of geological events these phenomena imply may be generalized 

 as follows :— 



1st. The deposition of the white clay and sand strata at some time since the 

 principal erosion of the mountain limestone, which took place about the Permian 

 period. 



2nd. The partial excavation of the pockets, and the gradual lowering into them of 

 portions of the previously existing deposits. 



3rd. The erosion of the great mass of the formation not protected in the pockets, 

 and the deposition of the boulder-clay drift. 



4th. The continued excavation and deepening of the limestone cavities after the 

 boulder-clay period, accompanied by the redepositing of the calcareous matter, 

 carried away in watery solution, as tufa. 



Deposits of a similar character occur in the mountain-limestone district of 

 Tipperary. 



On hitherto unrecorded Leaf Forms, §c, from Alum Bay, Isle of Wight. 



By W. S. Mitchell. 



On the Coal -Measures in Mold Valley, and their Products. By W. Ness. 



On Steam as the active Ayent in Earthquakes. By R. A. Peacock, Jersey. 



The author had collected seventy evidences of the presence of steam or its con- 

 stituents in every species of natural disturbance of the earth's crust. It was proved 

 by investigation of series z of M. Regnault's most excellent experiments *, that in 

 saturated steam of the temperatures 241 i° to 447° F., the pressure increased in the 



* Mcmoires de l'lustitut, vol, xxi. 



