162 MR CUNNINGHAM ON THE GEOGNOSY OF 



district, there is also the total want of that vitreous ap- 

 pearance which eminently distinguishes true lavas ; fur- 

 ther, in no instance do any of the basaltic or greenstone 

 rocks present unfilled cavities. We are aware that both 

 basalt and greenstone may be found in a cavernous state ; 

 such, however, is a mere effect of weathering, and en- 

 tirely disappears, if the rock is examined beyond the reach 

 of atmospheric agents. That this is a character by which 

 a Plutonic or trap-rock may be separated from a volcanic 

 formation, is stoutly denied by Dr MacCulloch. In his 

 System of Geology, vol. ii. p. 113, he states, that " It has 

 often been said by those who have written so much on 

 what they do not know, that the cavernous structure was 

 a test to distinguish between the trap and the volcanic 

 rocks. But it is not so, unless the Little Cumbray be sup- 

 posed the remains of a volcano. Had even the rock of 

 Edinburgh Castle been examined by those who wanted no 

 opportunities, this assertion could never have been made." 

 To this statement little is requisite to be said ; that autho- 

 ritative style which declares "It is not so," may, however, 

 be allowed to those who are of a different opinion from 

 this geologist ; and an examination of Edinburgh Castle 

 rock, as the other members of the trap series of Scotland, 

 will enable them to declare that "It is so," and that with 

 no small degree of confidence in the accuracy of their 

 statement. 



Of all the rocks of Eigg, the pitchstone is that which 

 approaches most to those of a true volcanic origin, both in 

 aspect and chemical composition. That this even has been 

 erupted under no inconsiderable pressure, is indicated from 

 the pumiceous and cavernous substance which it forms, 

 when it is subjected, without pressure, to a heat sufficient 

 to fuse it. Necker de Saussure, in his work on Scotland, 



