52 ANCIENT AND MODERN LAVAS COMPARED. [Ch XX r 



distinct idea will be formed of the dimensions of the I 

 streams^ if we consider how striking a feature they -wo i 

 now form in the geology of England, had they been poured 

 out on the bottom of the sea after the deposition, and hef 



r 



I 



CU' 



S^ 



the elevation of our secondary and tertiary rocks. 



The 



same causes which have excavated valleys through parts rf 

 our marine strata, once continuous, might have acted witl 

 equal force on the igneous rocks, leaving, at the same time 

 a sufficient portion undestroyed to enable us to discover 

 their former extent. Let us, then, imagine the termination 



> 



wide, might again 



of the Skapta branch of lava to rest on the escarpment of the 

 inferior and middle oolite, where it commands the vale of 

 Gloucester. The great platform might be 100 feet thick 

 and from 10 to 15 miles broad, exceeding any which can be 

 found in Central Trance. We may also suppose great tabular 

 masses to occur at intervals, capping the summit of the Cots- 

 wold Hills between Gloucester and Oxford, by Nortlileacli 

 Burford, and other towns. The wide valley of the Oxford clay 

 would then occasion an interruption for many miles ; but the 

 same rocks might recur on the summit of Cumnor and Shot- 

 over Hills, and all the other oolitic eminences of that district. 

 On the chalk of Berkshire, other tabular masses, 6 or 7 miles 



be found ; and, lastly, crowning tlie 

 highest sands of Higligate and Hampstead, we might beliold 

 some remnants of the current 500 or 600 feet in thickness, 

 causing those hills to rival, or even to surpass, in height, 

 Salisbury Craigs and Arthur's Seat. 



The distance between the extreme points here indicated 

 would not exceed 90 miles in a direct line ; and we might 

 then add, at the distance of nearly 200 miles from London, 

 along the coast of Dorsetshire and Devonshire, for example, 

 a great mass of igneous rocks, to represent the submarine 

 reef of the island of Nyoe. An eminent French writer 

 declared in 1829 that all geological phenomena took place 

 in ancient times on a scale of magnitude a hundredfold 

 greater than those which are witnessed in our days, but it 

 would be difficult to point out a mass of igueous rock of 

 ancient date (distinctly referable to a single eruption) which 



^10 





ei 



stup 







atar 

 thee: 



tbela 

 thes 



dista: 



suga: 



Cmt 



Ml 

 earl 



the 

 frac 



men 

 mil 



cones 



volca: 



se 



It 



inelu( 

 cease 



tHso( 



fter r 

 found 



en 





