74; 



fomiiiig the summits and flanks of Moel Hebog and Stlowdonia, are,- 

 he conceivess several thousand feet below the Bala limestone. 



The hope, however, whieh was entertained by my friend, of finding 

 these vastly expanded and loAver members characterized by pecu- 

 liar groups of fossilis has been frustrated, and whatever may be the 

 thickness of this lowest palaeozoic division, in which he has collected 

 a great number of species, he now fully admits, that zoologically it 

 is irom top to bottom a Lower Silurian series *. Charged as it is with 

 characteristic Orthidse and Trilobites, including the Asapkus tyran* 

 nus, so characteristic of the lowest Silurian rockj, there are, as 

 might be expected, a few new and undescribed species ; and, among 

 these, an Ophiura (an animal whose remains had not previously 

 been found in strata of higher antiquity than the Lias) will not 

 appear the least extraordinary. 



The base of the palaeozoic deposits^ as founded on the distinc- 

 tion of organic remains, may now therefore be considered to be 

 firmly established; for the Lower Silurian type is thus shown by 

 Professor Sedgwick himself to be the oldest which can be detected 

 in North Wales, the Country of all others in Europe in which 

 there is a great development of the inferior strata. But if classifica- 

 tion is settled, there still remains much to be done before North 

 Wales can be as accurately laid down upon a map as the parts of 

 South Wales to which I will presently allude; though when the 

 operations of the Ordnance surveyors are extended to this com"- 

 plieated region, we shall learn, by distinct geometrical admeasure* 

 raent, the exact thickness of these subcrystalline rocks on the flanks 

 of Snowdon. ; . ■ .■ . ..., ■ 't 



In reference to the terms Caradod Sandstone-and Llajadeilo Fl&gsj 

 as occasionally applied to the divisions of the Lower Silurian rocksj 

 I must express my dissent from a proposal made by Mr. Sharpe 

 to strike out one of these names from British nomenclature. He 

 believes from what he has seen, that on the whole the type is the 

 same throughout all these lower rocks ; but I think Mr. Sharpe 

 would not have offered this suggestion had ho surveyed the whole 

 of the Silurian region. If, for example, he had commenced his re- 

 searches where Sir Henry De la Beche has been accurately fixing 

 the limits of each forraation) he would at all events have admitted 

 the value of the term of Llandeilo flags, which the Director of the 

 Ordnance Survey has informed me he considers to be a remarkably 

 clear and well-defined formation, having a thickness of between 

 2000 and 3000 feet, and to be plainly separated from the Caradoe 

 sandstone. If Mr. Sharpe supposes that by such a term as Llan- 

 deilo flags, I intended to restrict this or any other formation to 

 a mass having one mineral character only, he has misapprehended 

 my meaning, and I refer him to a description of the strata, as seen 

 between Llandeilo and Llandovery, where sandstone, schist and 

 limestone, occurring in repeated alternation, form the "Llandeilo 

 flags." la the-first j-^ars of my researches in th-e Silurian regionj I 



* See an expression of the same opinions, Geoi. Proc. vol. iii. p. 5. p. 549. 



