124 OVERLYING AND TRAP ROCKS. 



manner, in the same mass, and even in the same 

 vein. 



Yet a succession of at least two, if not of more such 

 formations is proved, as in the former case, by means 

 of veins, traversing masses or intersecting other veins; 

 while I think that I have traced more than two such 

 sets of veins ; whence we can infer an equal number 

 of masses, or distinct productions of trap. A different 

 proof of such a succession of productions occurs in 

 Canna, where beds of trap alternate with strata 

 of a conglomerate containing rounded nodules of si- 

 milar materials. These, it is plain, must have been 

 derived from previous masses of a similar rock; while 

 it is equally evident, that there must have been an in- 

 terval of time between the two, to produce the attri- 

 tion and accumulation in question. The last proof of 

 this succession of trap at distant intervals is that, al- 

 ready noticed, where the lowest sandstone contains, 

 among other materials, many varieties of this family, 

 such as greenstones, porphyries, clay stones, and amyg- 

 daloids ; while, being covered by masses of trap, two 

 productions, and of extremely remote date, are thus 

 proved. But further, as both in this case and in that 

 of Canria, the superincumbent masses are traversed by 

 trap veins, there is absolute evidence of three produc- 

 tions at distinct periods, should the triple set of veins 

 in Airdnanmrcban not be satisfactorily demonstrable. 

 If the former indispensable general remarks on the 

 Unstratified rocks have anticipated much of what I 

 cannot nevertheless avoid noticing in the histories of 

 the different families, though under some repetition, 

 the two last cases are those whence I deduced a most 

 important conclusion respecting the repetitions of pro- 

 ductions of trap in one place. The trap conglome- 

 rates of the red sandstone series, are limited to certain 



