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 ; 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 



m 



