178 Dr. Mac Culloch's Supplement to the 



the trap. If they are disjointed in position, or if they appear pro- 

 miscuously scattered, they still retain their natural connection, 

 while the identity of their mineral structure is every where con- 

 sistent. In one place only some strata of a quartz rock are to be 

 seen, which might lead us to hesitate did we not recollect that 

 in other instances the same causes which have converted shale into 

 siliceous schist have also been found to change sandstone into quartz. 

 The same causes which formerly prevented me from examining 

 the strata of Trotternish, the deficiency of which I have now sup- 

 plied, also impeded the investigation of the coal which is connected 

 with them. Although I have since followed and traced the appear- 

 ances of this mineral in those places where it has been observed, 

 there is but little satisfactory information to be obtained respecting 

 it. The cause of this obscurity is easily understood. It has been 

 remarked that although the basis of this promontory consists of the 

 stratified rocks which have been just described, the whole is sur- 

 mounted and intersected by trap. The decomposition of this rock, 

 and that of the softer strata which lie beneath, have moreover 

 covered the whole country with a deep soil, which from its fertility 

 tends further to conceal the nature of the rocks on which it reposes. 

 Hence it is only in the casual exposure of some jutting rock or 

 broken face, some denuded acclivity or bed of a stream, that any 

 access can be procured to the stratified substances, and from this 

 cause it is rarely, if ever, possible to trace the relations of the par- 

 ticular stratum which comes into view. It is among such dispersed 

 portions of strata that the appearances of coal are observed. They 

 are not unfrequent, but are always extremely scanty, both in their 

 thickness and in their apparent horizontal extent, since the strata 

 which contain them are every where cut off by veins or by masses 

 of trap. They are interposed, as we might expect, among the 



