320 n.LOGICAL GEOLOGV. 



Up to 1839 it was inferred, from their crystalline char- 

 acter, that the metamorphic rocks of Anglesea are more 

 ancient than any rocks of the adjacent main land ; but it 

 has since been shown that they are of the same age with the 

 slates and grits of Carnarvon and Merioneth. Again, slaty 

 cleavage having been first found only in the lowest rocks, 

 was taken as an indication of the highest antiquity : whence 

 resulted serious mistakes ; for this mineral characteristic 

 is now known to occur in the Carboniferous system. Once 

 more, certain red conglomerates and grits on the north-west 

 coast of Scotland, long supposed from their lithological as- 

 pect to belong to the Old Red Sandstone, are now identifi- 

 ed with the Lower Silurians. 



These are a few instances of the small trust to be placed in 

 mineral qualities, as evidence of the ages or relative posi- 

 tions of strata. From the recently-published third edition 

 of Siluria^ may be culled numerous facts of like implication. 

 Sir R. Murchison considers it ascertained, that the siliceous 

 Stiper stones of Shropshire are the equivalents of the Tre- 

 madock slates of North Wales. Judged by their fossils, 

 Bala slate and limestone are of the same age as the Cara- 

 doc sandstone, lying forty miles off. In Radnorshire, the 

 formation classed as upper Llandovery rock, is described 

 at diflerent spots, as " sandstone or conglomerate," " impure 

 limestone," " hard coarse grits," " siliceous grit " — a consid- 

 erable variation for so small an area as that of a county. 

 Certain sandy beds on the left bank of the Towy, which 

 Sir R. Murchison had, in his Silurian System^ classed aa 

 Caradoc sandstone (evidently from their mineral characters), 

 he now finds, from their fossils, belong to the Llandeilo for- 

 mation. Nevertheless, inferences from mineral characters 

 are still habitually drawn and received. Though Silw%a, 

 in common with other geological works, supplies numerous 

 proofs that rocks of the same age are often of widely-dif- 

 ferent composition a few miles off, while rocks of widely 



