Rocks. 6 1 



ably, and fossils discovered in the north of Scotland by 

 Mr. Peach prove that Lower Silurian rocks (somewhat 

 metamorphic) rest un conformably on both. 



Till within the last few years it was customary to 

 consider that all formations which had not yielded 

 fossilised fresh-water shells were of marine origin. Mr. 

 Godwin-Austen first broke through this fallacy, and 

 often insisted that the Old Eed Sandstone, as distinct 

 from the Devonian rocks, was deposited in fresh-water 

 lakes. 



In 1871, I published two memoirs in the Journal 

 of the Geological Society, in which I attempted to 

 prove that in a broad sense, the red formations of 

 Britain were deposited in lakes, salt or fresh, or in 

 inland areas in which salt and fresh water alternated. 

 In one of these, 1 I ventured to state ' that the absence 

 of marine mollusca in the Cambrian rocks ' of North 

 Wales and the Longmynd, as yet observed, may be due 

 to the same cause that produced their absence in the 

 Old Eed Sandstone (see p. 106), and that 'the presence 

 of sun-cracks and rain-pittings in the Longmynd beds 

 favours this suggestion.' The occurrence of marine 

 fossils, chiefly in the grey slates and ' olive-green grits 

 and shales ' of St. David's, as described by Mr. Hicks, 

 ' may/ I state, ' possibly mark occasional influxes of the 

 sea into inland waters, due to oscillations of level,' pro- 

 ducing the same kind of alternations of marine and 

 fresh-water strata that occur in the Carboniferous series, 

 and in the Miocene beds of Switzerland and the Rhine. 



It is but right to state, however, that, as regards 

 the Cambrian rocks, mine is not the usual opinion. 



The LOWER SILURIAN rocks which conformably 



1 On the Bed Kocks of England of older date than the Trias. 

 March 1871, ' Journal of the Geological Society,' vol. xxviii. 



