296 PEOCBEDTNGS OF THE GEOLOGICAL SOCIETY. [^pr. 5, 



parts. The abundant vertical joints only rarely assume a prismatic 

 or rudely columnar arrangement. Hence, though it would often be 

 difficult or impossible to discriminate the intrusive from the inter- 

 bedded rock in hand-specimens, the distinctions between them are 

 well maintained when we have a cliiF-section before us. This 

 similarity and difference become readily intelligible when we regard 

 the two forms as in reality and originally proceeding from the same 

 source, their distinction being due to the different conditions under 

 which they were respectively consolidated ; and we then perceive 

 why the intrusive sheets should lie chiefly at the base of the inter- 

 bedded series. The former are portions of the Tertiary lava which, 

 unable to force their way to the surface, escaped laterally along the 

 lines of least resistance. The increasing mass of the great over- 

 lying sheets of the plateau would oppose more and more the rise 

 of the fluid lava, save from the main volcanic vents. Such por- 

 tions of the latter as were driven up through cracks would often 

 meet less resistance in trying to force their way along the bedding- 

 planes of the secondary strata, or between these strata and the 

 overlying igneous series, or between the lower beds of that series, than 

 in breaking through the thick and compact volcanic mass above. 

 Hence it is that, in nature, the intrusive sheets are in reality 

 found where we might expect to meet with them. These state- 

 ments involve, no doubt, only the most elementary knowledge. 

 Yet the want of a due appreciation of this knowledge, and of its 

 application in the field, has led to grave misconceptions as to the 

 age of the volcanic rocks of the Inner Hebrides — misconceptions in 

 which I have myself fully shared. I am naturally anxious, there- 

 fore, to point out that, while their intrusive relations have been 

 fully recognized, the pseudo-interbedded character of the intrusive 

 sheets at the base of the great basaltic plateaux of our west coast 

 has been confounded with the true interbedded character of the 

 sheets forming the plateau above, and that hence the inference 

 regarding the intercalation of contemporaneous volcanic rocks among 

 the secondary rocks of the Hebrides is without foundation. There 

 is no evidence of any truly contemporaneous volcanic rock, so far as 

 I have yet ascertained, in any of the Liassic or Oolitic rocks of 

 that region. The basalt-plateau, viewed as one great sheet, rests 

 alternately on Cambrian, metamorphosed Lower Silurian, Liassic, 

 Oolitic, and Cretaceous rocks, and unconformably upon them all, 

 from Antrim to the north of Skye. Here and there, where it hap- 

 pened to be laid down upon more or less horizontal strata, it shows at 

 its base intrusive sheets which seem to run parallel with it, as well 

 as with the secondary strata, between which they have been thrust. 

 And thus has arisen the apparent gradation of the Oolitic groups of 

 Skye into an upper volcanic series — a gradation, however, which is 

 quite deceptive, and which disappears when, after wider examina- 

 tion, we come to recognize the true intrusive character of the inter- 

 calated sheets, and the real unconformability of the basalt-pla- 

 teau alike upon Palaeozoic and Secondary formations*. 



* The suggestion of Edward Forbes regarding the probable Oolitic date of the 



