1844.] 297 



alteration is manifested in all the rocks around, nor for their un- 

 equally altered condition, amounting in some instances to indica- 

 tions of active fusion ; nor will it explain the occasional spots and 

 irregular forms of trap which are insulated in the limestone. 



Referring to the patch of limestone entangled in the northern- 

 most mass of trap, on the eastern side of the cutting, the author 

 states his conviction that no fragment of a similar limestone, of 

 like forms and dimension to that in question, could be separated 

 from its parent rock by mechanical agency of any kind, unless 

 by the most elaborate design ; so thin are the plates and so slender 

 are the strings of this limestone patch, which is insulated in the 

 trap. 



The explanation of these phenomena which the author has to 

 offer is, that the lime-rocks have been reduced in situ by tranquil 

 fusion, and have been converted by subsequent cooling and crys- 

 tallising into the trap which now replaces them, occupying the 

 very position before occupied by the limestone. 



The patch of limestone insulated in the trap, the author con- 

 ceives to have been a calcareous fragment, which had escaped 

 entire fusion ; and he can imagine no process in nature to account 

 for the spots of trap, insulated in the limestone, except that of 

 intense heat. 



The author notices that there is nowhere to be met with, in the 

 proximity of the trap, any extraneous matter which might be 

 referred to the action of liquid lava on the limestone ; but it is all 

 limestone, or all trap. What, he asks, can have befallen the 

 portions of the limestone beds which have disappeared, unless they 

 have been melted up, and converted into trap ? 



The author does not mean to question the fact that, in certain 

 instances, trap and other volcanic rocks have been forcibly intro- 

 duced among sedimentary and other rocks ; but only wishes to show 

 that, in other instances, there is evidence that the presence of trap 

 is attributable to the reduction of pre-existing rocks by volcanic 

 agency. 



These views led the author to arrange Volcanic Products under 

 two heads, the immediate and the intermediate ; the former consisting 

 of such products as have been fused down and crystallized in situ, 

 or have been ejected from submarine vomitories, or subaerial 

 craters ; the latter, of such rocks as have been acted upon more or 

 less extensively, and only partially reduced by active fusion, and 

 were therefore in different states of reduction when the temperature 

 was finally withdrawn. In the former he comprises granite, 

 porphyry, the several varieties of trap, tufaceous ash, breccia, 

 grit, flinty chlorite, talcose and clay slates ; in the latter head he 

 comprises gneiss and mica-, chlorite-, and hornblende- schist. 



z 2 



