PRIMITIVE ACTIVITIES 41 



that the rate has varied from time to time, but in what 

 direction, or to what extent, it is impossible to conjecture. 

 That the sun was once much hotter is probable, but 

 equally so that at an earlier period it was much colder ; 

 and even if in its youth all the activities of our planet 

 were enhanced, this fact might not affect the maximum 

 thickness of deposits. An increase in the radiation of 

 the sun, while it would stimulate all the powers of 

 subaerial denudation, would also produce stronger winds 

 and marine currents ; stronger currents would also result 

 from the greater magnitude and frequency of the tides, 

 and thus, while larger quantities of sediment might be 

 delivered into the sea, they would be distributed over 

 wider areas, and the difference between the maximum 

 and average thickness of deposits would consequently be 

 diminished. Indications of such a wider distribution 

 may, I think, be recognised in the Palaeozoic systems. 

 Thus we are compelled to treat our rate of deposition as 

 uniform, notwithstanding the serious error this may 

 involve. 



The reasonableness of our estimate will perhaps best 

 appear from a few applications. Fig. 3 is a chart, based 

 on a map by De Lapparent, representing the distribution 

 of land and sea over the European area during the 

 Cambrian period. The strata of this system attain their 

 maximum thickness of 12,000 feet in Merionethshire, 

 Wales; they rapidly thin out northwards, and are absent 

 in Anglesey; scarcely less rapidly towards Shropshire, 

 where they are 3,000 feet thick ; still a little less rapidly 

 towards the Malverns, where they are only 800 feet 

 thick ; and most slowly towards St. David's Head, where 

 they are 7,400 feet thick. The Cambrian rocks of Wales 

 were in all probability the deposits of a river system 

 which drained some vanished land once situated to the 

 west. How great was the extent of this land none can 



