Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/188

 the trap. If they are disjointed in position, or if they appear promiscuously scattered, they still retain their natural connection, while the identity of their mineral structure is every where consistent. 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 supplied, also impeded the investigation of the coal which is connected with them. Although I have since followed and traced the appearances 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 surmounted 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 particular stratum which comes into view. It is among such dispersed portions of strata that the appearances of coal are observed. They are not infrequent, 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