SKY. GEOLOGY. OVERLYING ROCKS. 381 



This rock may without inconvenience be designated 

 by the term augit rock, a name absolutely wanted if, aa 

 I have already said, it is found to constitute a principal 

 member of the multifarious family of trap. It is inter- 

 esting to remark that the characters of the lavas and 



o 



the traps are thus still further approximated, since the 

 observations of mineralogists have recently proved that 

 augit and not hornblende forms the dark part of these vol- 

 canic products. 



One circumstance relating to the position of the trap 

 is worthy of remark before proceeding to consider its 

 interference with the stratified rocks. In some places 

 the continuity of the uppermost incumbent stratum is 

 broken ; a single patch of columnar rock being found 

 lying on the strata detached from the surrounding 

 parts.*' This phenomenon assists in illustrating that 

 much greater loss of substance which in other cases, as 

 in that of Dun Can in Rasay, has left a single eminence 

 of trap insulated on the surface of the subjacent rocks. 

 The great tendency of this substance to decomposition, 

 explains the mode in which the loss of masses so exten- 

 sive takes place, without the necessity of having recourse 

 to violent causes of denudation. Other similar instances 

 have been noticed in different parts of this work, but 

 I may here point out a very remarkable example on 

 account of its accessible nature. It lies at Xhe north 

 Queen 7 s-feriy, the complete transition from trap to yellow 

 clay being undisturbed ; the marks of the rude columns 

 and of the spheroidal crusts by which they have succes- 

 sively decomposed still existing in the clay, though now 

 reduced to powder. 



mere exposure to the atmosphere. The travelling apparatus to which a 

 geologist is limited does not easily admit of the means requisite to ascer- 

 tain the proportion of water on the spot, where alone it could be 

 determined ; and the delay of even a day or two is sufficient to dissi- 

 pate it, in part or altogether. 

 * Plate XVII. fig. 3. 



