XXXIV PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



cussed ; they are already in part published, and the rest will shortly 

 appear in our 'Journal.' 



The attention of geologists has of late years been more habitually 

 directed to a careful and minute examination of the phaenomena of 

 modern changes ; and I believe that our theories of the appearances 

 which the older parts of the earth's structure exhibit will be sound 

 in proportion as they are in accordance with the laws which we see 

 governing the modifications that are now in progress. It is in this spirit 

 that Sir Henry De la Beche has devoted nearly twenty introductory 

 pages in his late valuable Memoir, ' On the Formation of the Rocks 

 of South Wales and South-Western England/ to an explanation of 

 existing causes of terrestrial change, in order that his reasonings on 

 the details he is about to describe m.ay be more intelligible and satis- 

 factory. " It may be advisable " he observes, " before we inquire 

 into these facts, to take a brief view of ' 3 effects of igneous action, 

 and of the deposit of mineral mato. , oiemically or mechanically, 

 from or by means of seas, estuaries, and 1.- -^s, as they are at present 

 known to us. By comparing this knowledge with the geological facts 

 observed in the district, we shall see how it enables us satisfactorily 

 to account for them, and how far other reasoning may be necessary." 

 As an example of his application of this mode of reasoning, which 

 indeed pervades the whole memoir, I will give the following instance ; 

 he is describing a hard indurated slate in the lowest parts of the 

 Silurian system, containing graptolites, and, according to the analysis 

 of Dr. Playfair, so much as five per cent, of carbon, and says, "It 

 would appear that black mud was a common sediment of the time, 

 the colour chiefly due to carbon, which we might infer was derived 

 from vegetable matter. Of what kind that vegetable matter may 

 have been, there would appear no direct evidence ; and though we 

 might be disposed to infer that marine plants may have furnished a 

 large part of it, when we regard the quantity of fine sediment of the 

 time and its extent, we should look, in accordance with the mode in 

 which such sedimentary matter is furnished in the present day, to 

 land, its disintegration, and its removal by rivers and running waters 

 to the sea, as among the chief sources of the non-carbonaceous part 

 of this black mud. Hence, and considering the conditions imder 

 which the remains of plants are likely to be preserved, it would pro- 

 bably be premature to look more than to plants generally, not alto- 

 gether excluding animal matter, for the carbon required." Thus he 

 contemplates, with every degree of probability, the existence of a 

 continent, intersected by rivers, clothed with vegetation, and subject 

 to atmospheric sources of disintegration as our present continents 

 are, at that distant period when the materials of the oldest of our 

 sedimentary rocks, the base of our vast series of geological forma- 

 tions, were accumulating at the bottom of a sea. 



The lowest stratum of the lowest sedimentary deposit constitutes 

 the limit beyond which we cannot trace the operation of those agen- 

 cies which are still modifying the structure of the earth ; it is the 

 true starting-point of all our speculations into the past history of the 

 globe that rest on authentic evidence ; there the proper work of the 



