504 GENERAL COMPARISON OF 



will be found. The apparent difficulty may arise solely 

 from our ignorance of the changes which the surface has 

 undergone. If the whole of the primary strata, for 

 example, was once covered by the secondary, no portion 

 of trap could now be found without being associated with 

 the latter ; since it is a rock of more recent origin. If a 

 minor degree of the same order of things be conceived, 

 it also follows that the trap rocks will proportionally be 

 found prevailing above the secondary strata, and that this 

 association does not indicate any necessary connexion 

 between the two, but an incidental one only, due to the 

 originally extensive spaces occupied by those strata. 



It remains lastly, in comparing the strata of the several 

 islands of the Clyde, to inquire if any connexion can be 

 traced between the portions of coal occurring in Bute and 

 Arran, and those of the adjoining mainland. So little 

 of this substance is found in these islands, and the positions 

 in which it occurs are so peculiar, that it is scarcely 

 possible to institute such a comparison. 



It is well known that an extensive tract of strata con- 

 taining coal, reaches from the Clyde to the eastern sea. 

 The general characters of this series are very simple when 

 compared to those which appertain to the coal districts 

 of England, but it is out of the bounds of this investiga- 

 tion to enter into their history. It is now necessary to 

 remark, that on the western side of Cantyre, near Mach- 

 rianish bay, a small portion of coal occurs, accompanied 

 by the same strata that prevail in the great tract above 

 mentioned, and bearing the same relation to the red 

 sandstone. The linear prolongation of this tract to 

 that one, passes through Bute and Arran, so as to include 

 the coal of both these islands ; thus appearing to indicate 

 the same general continuity of this substance as of the 

 red sandstone on which it lies. Here, however, the evi- 

 dence of this continuity ceases ; as the same accompanying 

 strata and the same regularity do not exist in these islands. 

 On the contrary, it has been seen that the coal of Arran 



