1846.] SEDGWICK ON THE FOSSIL SLATES OF N. WALES, ETC. 135 



rocks under notice. At the very least, ninety nine hundredths of the 

 trappean rocks associated with the system of Cambria are contem- 

 poraneous ; consisting of tabular masses of erupted porphyry (of 

 which the actual centres of eruption are seldom exposed to view), 

 of brecciated masses, of plutonic fragments, of trappean shales 

 [schaalstein), of recomposed porphyries, &c. &c., alternating with, and 

 passing into, the regular slate rocks, and sometimes containing or- 

 ganic remains. But in all this complicated series I know not a single 

 instance (either in North or South Wales) of any unequivocal vol- 

 canic or subaerial product *. The series is essentially subaqueous 

 or plutonic. 



These singular masses form an integral part of almost every sec- 

 tion. They often mark the great Cambrian epoch as perfectly as 

 the subordinate fossils (a remark which applies both to Cumberland 

 and Wales), and they belong so essentially to almost every import- 

 ant section, that they may be regarded as a part of the very alphabet 

 in which nature has, at least in our island, written her older records. 

 Without comprehending the part they have played, it is impossible 

 to understand the thickness of the whole series, the nature of its de- 

 velopment, or the relative position of the several physical groups. 

 In addition, I may remark (what I have often stated before), that the 

 contemporaneous plutonic rocks partake of common movements, and 

 have the same anticlinal and synclinal lines, with the older slates of 

 Cambria ; a fact in itself almost decisive on the question of the ge- 

 neral contemporaneity of the aqueous and igneous rocks here no- 

 ticed. 



Sections through the older rocks of North Wales. 



From the Menai Straits to the Berwyn chain the older rocks are 

 thrown (as I have often before described them) into a series of great 

 undulations, of which the anticlinal and synclinal lines nearly, though 

 not exactly, coincide with the strike of the principal mineral masses, 

 whether slates or any of the contemporaneous igneous rocks above- 

 mentioned. Thus, for example, the great porphyritic masses which 

 range southwards from Arrenig — the range of Arran Mowddwy and 

 Cader Idris — the enormous masses of porphyry east of the crest of 

 Snowdon, and extending thence down to Pont Aberglaslyn, &c. 

 &c., are all contained within the anticlinal and synclinal lines of a 

 series of nearly symmetrical undulations. To suppose that any one 

 of these ridges (e. g. the great ridge of Cader Idris) is an erupted 

 mass of a date posterior to the epoch of the slates, is so untrue to 

 nature as to destroy the value of any section involving such an error. 

 On this point, I need not inform my hearers, Mr. Sharpe and I are 

 at issue. I have not the shadow of a doubt about the general truth 

 of my views ; and a little army of good observers is now marching 

 over the country, who will settle points of difference, and give a 

 finish to details, utterly beyond the physical powers of one who 



* Some of the igneous products of Devonshire {e. g. Brent Tor) appear to 

 have been subaerial. In such cases the earthy beds made out of the igneous 

 products may very conveniently be designated by the term ash, so commonly used 

 by Sir H. De la Beche ; but I should reserve this term to cases like that jusfc 

 quoted, and not applv it to any plutonic products. 



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