SKY. GEOLOGY. OVERLYING ROCKS. 369 



common stratified trap, since the same difficulty occurs 

 in attempting to connect it with that mountain mass* 

 It is true, that while the main body of the syenite is 

 found reposing on the limestone, the hypersthene rock 

 of the Cuchullin lies on the red sandstone at the only 

 place where its junction with the stratified rock is visible. 

 It is thus apparently connected with an older rock ; but 

 if the unstratified rocks are throughout of a later date 

 than the stratified, of which the evidence appears 

 unquestionable, this circumstance must be considered as 

 accidental and as incapable of proving any thing respecting 

 their relative periods of formation.* 



* From laying too much stress on these connexions, and from theoretic 

 views of the nature of the unstratified rocks of this family, have arisen 

 distinctions which are often merely nominal, of traps of one or of 

 another age, of first, second, third, and newest formations. The same 

 mass will often possess the characters of all these, since it will in one 

 place be found incumbent on the latest, in another on the most ancient 

 rocks. I know not indeed that any certain marks of distinction 

 can be pointed out among the several traps. In the greater number 

 of instances at least which have fallen under my examination, such a 

 relation between an unstratified rock, occupying the irregular and 

 ^uncertain position which trap does, and those regular strata which 

 maintain a constant order of succession, is by no means a criterion from 

 which to judge of its relative order with regard to these. The aid of a 

 diagram is perhaps required to render this statement intelligible to those 

 who have not examined the rocks. The same mass of trap will for ex- 

 ample be found in one place incumbent on clay slate, in another on red 

 sandstone, in a third on limestone, in a fourth on the uppermost secondary 

 strata. In such a case, when the separate portions are either not all acces- 

 sible, or when they lie far distant and interruptedly, we might be easily led 

 to conclude that they were so many distinct deposits, and thus apply to 

 them terms derived from the particular beds with which they were found 

 immediately in contact. A more intimate acquaintance with such masses, 

 and with the general nature of trap, is required to correct these erroneous 

 conclusions, and the accompanyingf sketches will explain that which 

 actually occurs in nature, and of which, if I mistake not, instances are 

 to be found in many situations in Scotland. Cases, doubtless, exist 

 where a real distinction of periods in trap rocks can be proved ; but 



t Plate XVIII. fig. 5, 6. 

 VOL. I. B B 



