AND STRUCTURE OF THE BELEMNITE. 395 



admixture of clay along with shells. The upper chalk is dis- 

 tinguished by its inferior hardness, and the abundance of flint 

 with which it is accompanied. 



In Ireland, excepting when traversed by dikes, 1 have never 

 observed but one kind ; it is harder than any of the English 

 chalks ; it contains quantities of flints, and, as we shall shortly 

 see, a tolerable abundance of organic remains. It is used for 

 all the different purposes of limestone, including building. 



The alteration produced by the occurrence of a whin-dike 

 has been observed by every geologist who has visited this part 

 of the country. In place of the dull earthy fracture of the 

 chalk, it presents a crystalline disintegrated mass, very friable 

 and phosphorescent, of a pale green colour, and maintain- 

 ing these characters only in the immediate vicinity of the 

 dike. 



Geologists seem to have been very shy in treating of the 

 formation of chalk j they have in general been contented with 

 quoting the opinions of some predecessor, very little that 

 can be considered original, being to be found in any of the 

 geological works I have examined. Some ingenuity has, how- 

 ever, been displayed in ringing the changes on the same ideas ; 

 but in the progress of this investigation, we find very little 

 added to the first projected opinions. Professor Jameson sa- 

 tisfies himself with placing the Chalk at the end of his great 

 Limestone series, (System, p. 91. 1818), and considers that it 

 agrees admirably with the preconceived ideas of the diminution 

 of the waters, with which Werner inundated the surface of the 

 Earth ; and he adds, that its occurrence on the sea-coast, and 

 its earthy aspect, point out the lateness of its formation, (176). 

 Why the proximity of the sea-coast should afford any evi- 

 dence of the period of its formation I cannot conjecture, as we 

 know of no rock that is not washed by the waves of the ocean. 



Mr 





