191? Rev. W. D. Conybeare on the Phenomena of Geology 



described, containing only very small pebbles, such as we may 

 consider at all times to be pretty generally distributed over 

 the channel of the ocean. Now this absence of large water- 

 worn fragments, as well as the circumstances attendant on the 

 instances of their presence, appears to me to furnish data in- 

 dicating the true theory of their origin ; for otherwise, it 

 might be said that the ordinary action of the waves on the sea 

 coast was quite adequate to explain the existence of these 

 gravel beds, without any necessity of having recourse to ex- 

 traordinary diluvial currents. But if so, why do we not find 

 them universally intermixed among our formations ? for at 

 every period there must have been lines of sea coast, and the 

 ordinary action on these coasts must, of course, have been 

 constant and uniform. Why, on the contrary, do we only find 

 them among the products of periods which, on independent 

 grounds (the dislocation of the strata), we must conclude to 

 have been epochs of extraordinary convulsions, and of such 

 convulsions as we cannot conceive to have taken place, with- 

 out having been accompanied by much disturbance in the 

 level of the then existing oceans, and, consequently, by vio- 

 lent diluvial currents ? Thus, during the deposition of the 

 oolites and chalk (as we have already observed in the first 

 part of our essay), we observe few, and comparatively trifling 

 indications of the operations of the dislocating forces : and 

 here, also, we find few and trifling indications of diluvial cur- 

 rents. But, as we have already seen, in the tertiary period the 

 convulsions that elevated the strata of the Isle of Wight and 

 the Dorsetshire coasts, and those which elevated much of the 

 Alps, took place ; now we may well represent to our minds 

 what must have been the effect of these convulsions on the sea 

 level, if we should, for a moment, endeavour to imagine what 

 would be the consequences of their repetition. Supposing, 

 for example, that a new island, 800 feet high, were suddenly 

 to be protruded from the bosom of the sea on the Lincolnshire 

 coast, and that, at the same time, 60 miles of the adjacent flats 

 on that coast were broken up, and their beds thrown from an 

 horizontal into a vertical position, what, in such a case, must 

 be the agitation of the waters ! would not the resulting flood 

 spread far and wide ? and may we not conceive that the diluvial 

 waves would overtop the neighbouring chain of the Wold 

 hills, scoop out deep valleys in them, and reduce much of their 

 materials to the state of gravel ? 



The gravel associated with the plastic clay is principally 

 composed of flints derived from the chalk strata : it may be 

 well studied in the neighbourhood of London, as it underlies 

 all the elevated grounds of the plain of Blackheath ; and may 



be 



