74 



OUTLINES OF GEOLOGY 



Schorl, 



Mesotype, and all the 



Prehnite, 



Amethyst, 



Jasper, 



Harmotome, 



Calc-spar, 



Arragonite, 



Felspar, 

 )litic minerals, 

 Chabasie, 

 Agates, 

 Opal, 



Sulphate of Strontia, 

 Sulphate of Baryta, 

 Native copper. 



171. The rocks of this order occur principally in dykes, 

 which give birth to intercalated masses ; they also occur in 

 overlying masses, aspiring to the dimensions of mountains. 

 They do not occur intercalated in any formations lower than 

 the medial order, and seem to be limited on the other side by 

 the diluvium. The most important rocks of this order are 

 those which traverse, in the form of dykes, many of the coal 

 fields. By these dykes the basins are divided, as it were, 

 into many separate chambers ; and were it not for the ob- 

 stacle which these oppose to the filtration of water, it would 

 have been impossible to have drained many of the masses of 

 that useful fossil. 



ITZ. Among the most celebrated formations of this order 

 are those of the Giants' Causeway, in Ireland, and Fingal's 

 Cave, in the Isle of StafFa. In these localities the fissures 

 give to the rock the form of polygonal prisms, and these are 

 often nearly regular in their form. Each pillar is composed 

 of blocks, having a concave and convex surface. 



The neighbourhood of New- York furnishes many interest- 

 ing localities of this group. On the opposite shore of the 

 Hudson is a ridge called the Pallisadoes, composed of colum- 

 nar trapp . This is first observed beneath the surface of the 

 water at Bergenport, and, gradually rising, reaches the 

 height of 600 feet, at Haverstraw. This ridge is traversed 

 by but a single pass, at Tappan. At Haverstraw the moun- 



