DISTINCTIONS BETWEEN VOLCANIC GRIT AND INTRUSIVE TRAP. 75 



trap rocks, which has been little adverted to by writers, and upon which I shall en- 

 deavour to throw some new light. These are the rocks named in the following chapters 

 " Volcanic grit," " Bedded and contemporaneous trap," and which I undertake to prove 

 were formed at the bottom of the sea during the accumulation of the sedimentary matter 

 with which they are associated, particularly in the lower strata of the Silurian System. 

 At one place these appear as currents or sheets of pure volcanic materials, at another 

 they envelope marine remains, pebbles, sand, and fragments of rocks. Some layers 

 consist of finely levigated volcanic scorise passing into sand $ and all these varieties 

 alternate so equally and repeatedly with beds composed exclusively of shelly and marine 

 sediments, that no doubt can be entertained that the diversified masses so arranged in 

 parallel strata, must have been formed during the same period of igneous action. In 

 the remote sera therefore of the Silurian System, the evidences of volcanic operations 

 are similar to those which Mr. Lyell has noticed in the modern deposits of Sicily, 

 where banks of existing species of marine shells, now at considerable heights above the 

 sea, are so interlaced with volcanic matter, that no other deduction can be permitted, 

 than that the whole of these masses were of contemporaneous submarine formation. 

 "We are therefore," adds that Author, " entitled to expect, that if we could obtain 

 access to the existing bed of the ocean, and explore the igneous rocks poured out 

 within the last 5000 years, beneath the pressure of a sea of immense depth, we should 

 behold formations of modern date very similar to the most ancient trap rocks of our 

 island 1 ." 



In pointing out an analogy to existing nature in the bedded trap of a very ancient geo- 

 logical period, we also fix the chronology of one class of igneous rocks ; — for those of 

 intrusive character so predominant in these islands, are rarely capable of such a limita- 

 tion. We have often heard, indeed, of " coal measure trap," and " greywacke trap," as 

 if such rocks were formed during the carboniferous or greywacke periods, whilst in the 

 great majority of examples nothing is further from the truth. For instance, it will pre- 

 sently be shown, that basalt has penetrated and overflowed coal-fields, dislocating and 

 fracturing the strata, and therefore we know it must have been erupted subsequently to 

 their consolidation : but as the carbonaceous strata so affected are not covered imme- 

 diately by any newer deposit, at what period were they so penetrated ? Examples, 

 indeed, will be given, from which it may be fairly inferred, that the coal-bearing strata 

 have been forced up through once overlying red sandstone, and therefore that some of 

 the volcanic agents which disturbed these coal-fields were in action subsequently to the 

 sera of the New Red System. In the same manner trap will be shown to have pene- 

 trated new red sandstone, and reasons will be given for legitimately concluding, that 

 such eruption took place after the completion of the New Red System, and even pro- 

 bably of the Lias. 



1 Principles of Geology, 4th edition, vol. iv. p. 254. 



