116 OVERLYING AND TRAP ROCKS. 



prove the identity between the trap rocks and tht 

 volcanic ones, and the common cause of both. 



The last member of this family is TUFF, or tufo ; 

 while, presenting some important differences of geo- 

 logical origin, I must here distinguish what geologists 

 have always confounded under a mineral term, with 

 the usual neglect of causes, or of geological philo- 

 sophy. There are conglomerates of trap under this 

 name, that are merely portions of the old red sand- 

 stone, in which, more or less of the casual materials 

 have been derived from antient traps. Geology re- 

 quires that they be separated, and ranked as varieties 

 of that sandstone. Again, it is not unusual at the 

 junctions of trap with the same sandstone, to find that 

 the fused rock lias invaded the conglomerate, while, 

 in altering some of the fragments, it has given to the 

 whole the aspect of a real trap conglomerate or tufo, 

 yet only in the eyes of those who cannot make the 

 distinctions which appear never yet to have been 

 made as to this case. If Geology is to mean what it 

 ought, this must also be rescinded from the trap tuffs, 

 while its place in the system is obvious. There re- 

 main then but two varieties that I know of, to be 

 truly placed under this title ; and both consist of 

 mixed fragments, of all sizes, rounded or not, with 

 trap sand, often so little adhering as even to leave in- 

 terstices, though these are sometimes filled with in- 

 filtrated carbonat of lime. Each kind is covered by 

 or alternates irregularly with solid trap, as they also 

 sometimes occur in apparent veins, further containing 

 occasional lignites, as is hereafter explained. Their 

 fusible nature, added to their mere existence as tufos, 

 proves, that however connected with solid traps, they 

 have not been subjected to much heat, while their 

 non-conducting power will explain their escape under 



