236 SECONDARY LIMESTONES. 



scurity; while geologists have increased the difficulty, 

 or rather, have created one, by attempting to find 

 identities that have no existence. Nor is it part of 

 this design to describe all the local variations of these 

 rocks and deposits, even where best ascertained; to 

 detail the successions and alternations of every bed of 

 limestone that has been described. Such information 

 belongs to topographical geology, not to a general 

 view of the Science ; while the enquiry would involve 

 discussions respecting many hypothetical statements 

 and analogies, better left in the hands of the great re- 

 former, Time. There has been a peculiar temptation 

 to extend analogies and identities in the case of these 

 limestones; because, independently of their impor- 

 tance, it has been supposed that they could be iden- 

 tified, even in countries widely separated, through 

 their organic fossils. I have shown, first a priori, and 

 then by evidence, that this test was often fallacious 

 or insufficient, and that the consequence must be error 

 or fiction; while it is certain also, that many lime- 

 stones are limited to particular districts. If there is 

 any one especially extensive, it is the series which 

 follows that very widely diffused deposit the red marl; 

 and, in a certain sense perhaps, both these may be 

 considered as general, but not " universal" deposits; 

 though the latter prevails in many countries where 

 the subsequent limestone does not exist. I shall 

 therefore continue to limit myself to the most con- 

 spicuous calcareous deposits, and chiefly to examples 

 from Britain; partly because they are best determined, 

 and partly from their peculiar claims on a British 

 writer: referring to authors for those details which 

 transcend my limits ; yet cautioning the student against 

 the overweening of those who imagine that in de- 

 scribing the topography and details of the English 



