8. All^jort — Tertiary and Palceozoic Trap-rochs. 197 



case, the augite being so abundant in the Antrim rocks as to form the 

 base, while in the Limerick traps it occurs in small quantities, and 

 has crystallized out in individual crystals, bearing a small proportion 

 to the whole mass. 



After stating that these facts point to an essential difference in the 

 two magmas. Prof. Hull obseryes that the distinction here noted 

 between the structure of the Carboniferous and Tertiary augitic rocks 

 is not admitted by me ; and concludes with the remark that his own 

 observations tend rather to confirm the opinion of Mr. Forbes, which 

 is, that eruptive rocks of different geological epochs differ essentially 

 in mineral constitution. 



I would observe, in the first place, that I certainly am not pre- 

 pared to admit that a difference in the relative proportions of any of 

 the constituents is sufficient to indicate an essential difi^erence in the 

 rocks in which they occur. My experience is, that there is frequently 

 so great a variety in the relative quantities, and even in the state of 

 crystallization of the minerals, in parts of the same rock, that I 

 doubt the existence of any considerable mass, which would not 

 sujDply differences quite as great as those pointed out by Prof. Hull. 

 Examples of such variations are quite common in the Carbonifer- 

 ous and Tertiary dolerites, and are well exhibited in the Antrim traps 

 themselves. I have twelve sections of these rocks, and although 

 neither of them could fairly be described as consisting of a base of 

 augite, containing felspar, etc., I do not in the least doubt that such 

 may occur ; for in one of them the augite is very abundant, irre- 

 gularly crystallized, and is frequently penetrated by felspar ; there 

 are, however, numerous felspar crystals forming the usual irregular 

 network, and although the augite presents no definite forms, it is 

 in- a perfectly crystalline state, as is evident from its action on 

 polarized light. Another specimen from Ballyclare is a good typical 

 dolerite, containing numerous well-formed crystals of felspar, with 

 grains and crystals of augite and olivine scattered through the mass ; 

 another variety contains far more olivine than augite ; and a fine- 

 grained basalt from the Giant's Causeway contains innumerable 

 small crystals and grains of augite, felspar, and magnetite in a glassy 

 base. 



This glassy base, which is said to constitute a difference between 

 the two series of rocks, certainly occurs in the Antrim basalts, and 

 is common in many Carboniferous and Tertiary dolerites ; it can there- 

 fore form no ground of distinction between the two. 



In the description of the Limerick traps the base is said to be in 

 all cases a glass, and the terms glass base and felspathic base are used 

 indiscriminately. It appears to me very desirable to make a clear 

 distinction between the two expressions ; for whatever may be the 

 chemical composition, of the substance properly called glass, it is 

 quite structureless, exhibits no trace of double refraction, and there- 

 fore always remains dark between crossed prisms. Some of the 

 felspar may also appear dark at the same time, but if the polarizer 

 is rotated a few degrees, and the two prisms again crossed by moving 

 the analyzer, the relative positions of the two sets of optic axes are 



