166 The great CoaUf.eld of North Wales, described. 



appear on the SE side of Bala Pool, NE of Dolgelly, and in se- 

 veral other parts of North Wales, or whether such are strata of 

 Limestone m the slate ? I have not at present the means of 

 knowing. 



I now, consider the identity of the Limestone and superincum- 

 bent Coal-measures of Flint, Denbigh and Salop Counties, and 

 those of Derbyshire and Yorkshire, as sufficiently made out, by 

 my recent Survey, striking as the dissimilarities of them appeared 

 at first sight, viz. in the substitution of such very diifercnt strata 

 in places of the Toadstones; the extraneous fossils proving 

 much rarer here timn in Derbyshire. But the s-pecics of those that 

 are found, will however agree in those distant places, 1 believe, 

 both of shells in the Limestone and 9th Coal-shale ; and the 

 Crowstone Reeds (W of Oswestry in jiarticular) and other coaly 

 vegetable Impressions. The Limestone-Shale also proves very 

 thin in most parts of North Wales, and either in or under 

 it, an unusual quantity of Chert occurs, locally, in thin strata 

 (sometimes striped as in Ashover and other jilaces in Derbyshire), 

 which siliceous beds and the Limestone near them, seem to be 

 the most productive of Lead Ore, Calamine* and Blende, and 

 perhaps none of the Mineral Veins bear Ore in the lower part of 

 the series, near to the Slate: blue pozolanic Limestone is locally 

 found, imbedded in the lower part of this Limestone Shale. The 

 Coal-shales most of them seem thinner than in Derbysliire; and 

 to the NW of Holywell, useful seams of Coal do not apj)car be- 

 low the 5th Grit Rock: but near Oswestry, on the contrary, the 

 scams in the 3rd Coal -shale appear more valuable than perhaps 

 any in this part of the series in or near Derbyshire. In the 10th 

 and 11th Coal-shales (if I mistake not the numbers in the loQal 

 patches of these, surrounded by Faults) five-yard and three-yard 

 seams of Coals occur, which are thicker than are known in Derby- 

 shire ; perhaps the great decrease here, in the thicknesses of the 

 intervening measures, may have brought different seams of Coal 

 in contact?. The Gritstone Rocks in the lower part of these 

 Coal-measures, are vastly thinner than those in the south of 

 Yorkshire, or even in any part of Derbyshire, I believe, and 

 coarse Gritstone is much more rare, NW of Holywell ; in one 

 spot only (I I m. SE of Llanasa) could I discover it, in the 1st 

 Rockj which Rock, in a great part of its range, appears here 

 only as a thin and imperfect gritstone, or as stone Bind beds. 

 The 4th Grit preserves here its usual superiority, as a paving- 

 stone Rock, and in several Quarries on Bryn Llystyn Hill S of 



* This is mostly of the siliceous kind, which is perhaps unknown in Dcr-r 

 bysliire?. 



G\vesbyT 



