CUMBHAV (LITTLE). GEOLOGY. 487 



imbedded nodules have been washed out, the interior of 

 the cavity presents a peculiar appearance ; exhibiting a 

 fibrous and somewhat botryoidal surface, often further 

 glazed with a slight vitreous varnish. 



In many cases the cavities are only encrusted with the 

 matter of the nodule, the larger portion remaining empty ; 

 while, in some, they seem to have remained in this latter 

 state from the commencement ; being found unoccu- 

 pied on breaking into those parts of the rock which are 

 excluded from air and water. The specimens which pre- 

 sent this character in a high degree, are conspicuous 

 among the others from their cavernous structure. It is 

 indeed often impossible to distinguish them from scori- 

 form lavas, so identical are their characters with those 

 of the volcanic rocks. Similar specimens occur among 

 the trap in many other places, but tfyey are rarely so 

 perfect. 



This is a subject of general and not of local interest 

 in the history of the trap rocks, and one that has given 

 rise to much discussion. It has been said that no instance 

 of a trap strictly cavernous exists, but that in all cases the 

 cavities are the result of the loss of those minerals 

 which previously occupied them when they were in the 

 form of amygdaloids. -But such cavities are found deeply 

 imbedded in solid rocks to which the weather can have 

 had no access, and under circumstances where water could 

 not have dissolved and removed an imbedded nodule. 

 Such is the case here in a very remarkable degree. 



That any other cause but the extrication of air should 

 have produced these cavities is highly improbable, and, 

 on the igneous view of the origin of these rocks, the 

 existence of such a cause is sufficiently proved. Whether 

 there is any necessity for admitting, generally, that the 

 rocks of this tribe have been in a iluid state under great 

 pressure, or not, it is here unnecessary to inquire; but 

 it is sufficiently plain that in this particular case, whatever 

 be the period and the place of fusion, the incumbent 



