364 SKY. GEOLOGY. OVERLYING ROCKS. 



incapacity for the task ; the little I have to offer towards 

 a new arrangement will be better reserved till the other 

 islands similarly constituted have been examined. In 

 the mean time it is impossible to proceed in the descrip- 

 tion of Sky without giving some account of the different 

 substances included under the form of trap, that will here fall 

 under review; which may be done in the most brief manner. 



The first of these is claystone, resembling, in its leading 

 characters and composition, the pale substance more 

 generally known by this name, but differing in the colour, 

 which presents various tints of grey or lead blue; that 

 colour indicating a different state, and at times a different 

 proportion, of the iron that enters into its composition. 

 It occurs in different states of induration, and may 

 occasionally be found passing into the clinkstone, and 

 into the compact felspar of mineralogists ; the colour 

 in these cases being neglected as an insufficient ground 

 of distinction. It is necessary to distinguish this substance 

 in its indurated states, from greenstone and from basalt, 

 with both of which it is occasionally confounded. It 

 will be found here to occur principally among the 

 mountainous irregular masses, less abundantly among 

 the stratified, although found in these and also in the 

 form of intersecting veins. 



The next substance in the order of comparative sim- 

 plicity is basalt, a rock as yet but ill defined, if indeed 

 it be capable of strict definition : it will be found to 

 occur almost exclusively among the stratified masses, but 

 sometimes also occupies veins. 



I know not if the soft argillaceous substance which 

 is the common base of the amygdaloids in this island 

 will be admitted among the claystones, or whether British 

 mineralogists are inclined to extend the name of wacke 

 to it; since it differs in many particulars from that rock 

 as it is known to the Germans. It differs equally from 

 the claystones in some particulars, and may perhaps for 

 the present be distinguished by the term of indurated 



