220 Dr. Mac Culloch on the Limites. 



cation of an analogous theory to the formation of common 

 stratified coal. It will be seen that they prove nothing on either 

 side ; but are in a great measure independent of the peculiar 

 circumstances under which the trap rocks have been in a state of 

 igneous fluidity. 



Lignite is often found in the conglomerates which accompany 

 the trap rocks, and which are commonly known by the name of 

 tufo or trap tuff. These consist of fragments, generally of trap 

 rock alone, but sometimes admitting other substances ; and they 

 are commonly of a tender texture, or imperfectly connected by 

 minute fragments and earth, or powder of the same nature. Such 

 conglomerates often contain rounded materials ; proving that 

 some time and some transportation, or motion, have taken place 

 in the parts previously to their consolidation. 



It is not difficult to understand how fragments of wood may 

 have been introduced among substances which have once been 

 on the surface of the earth in a loose state ; and how the whole 

 surrounding mass may have been consolidated, so as to include 

 them. It is very certain that the consolidation has not, in these 

 cases, resulted from fusion, or from any high degree of heat ; as 

 the very fusible sand which forms the basis of such conglomerates, 

 could not have borne such a heat, and have retained its loose 

 texture. I need not here enter into any details of the modes in 

 which solid trap may have been deposited on such tufaceous or 

 conglomerate strata, as it must already be understood from the 

 history of the trap rocks which has been given elsewhere. It is 

 certain that such crystalline traps do lie upon loose tufas ; and that 

 as these could not, from their nature, have been subjected to any 

 high degree of heat, the lignites which they contain, have, in the 

 same way, escaped the effects of fire. This fact, therefore, does 

 not prove that the solid trap rocks have not been in fusion, but 

 merely that the tufas have not been so formed ; of which, indeed, 

 no other evidence than their own structure is necessary. Neither 

 does it prove that the wood has been bituminized by the action of 

 fire ; from which it must on the contrary have been, like the sur- 

 rounding materials exempted; while the reasons of a chemical 



