152 MR CUNNINGHAM ON THE GEOGNOSY 01 



highly compact and lustrous, and which is rendered por- 

 phyritic by the presence of crystals of glassy felspar. The 

 fact of this pitchstone becoming more compact as it ap- 

 proaches the traversed rock, is, in its nature, similar to 

 those appearances which the greater number of veins and 

 overlying masses display at their junctions with stratified 

 or older deposits, and is to be referred to the same cause, 

 viz. to a quicker cooling of the rock, from being in con- 

 tact with one previously consolidated. This circumstance 

 implies that igneous action continued to exist in the dis- 

 trict for a considerable length of time, and that Plutonic 

 matters were discharged at least two epochs apparent- 

 ly distant from each other, and separated by a period suf- 

 ficient for the refrigeration of those bodies of greenstone 

 and basalt which form the greater part of the island. How 

 long these trap-rocks took to consolidate cannot be esti- 

 mated. If, however, we remember that a sub-aerial lava 

 has been found to retain a high temperature ten years 

 after its eruption, how much longer time must that rock 

 take which flowed over the bottom of a deep sea, or be- 

 tween previously elaborated strata? Both the veins of 

 pitchstone are shifted, thus evincing, that, even after their 

 eruption and cooling, disturbing causes had acted. The 

 largest vein of pitchstone is remarkable from being broken 

 through in its upper part by a rock of porphyritic compact 

 felspar, which contains, in several places, imbedded frag- 

 ments of the pitchstone ; it is, indeed, not at all improbable 

 that the outburst of the felspar has been contemporaneous 

 with the shifting which we have mentioned as occurring 

 here.* Plate V. fig. 1* Liquid bitumen fills, in several 



* Dr MacCullock, in his description of the "Western Islands, has de- 

 nominated this rock " Chert ;" our reason for not adopting the same 

 or a similar title was, that we could not find in this mineral any cha- 

 racter which could justify us in giving it a place in the quartz family. 



