April, 1894. CONTINENTAL GROWTH. 291 



one has a profounder regard than myself, any indication of how the 

 distinctive geological and physical features of the several periods 

 came about, or, indeed, why they should exist at all. As a student 

 of Lyell from my earliest dabblings in geology, this was long a perfect 

 mystery to me. In putting forward the following suggestions as to 

 how the periods were evolved, I do so with all humility, looking upon 

 them as a development of the great master's work. 



Sedimentation and Land Making. 



It is a well-known axiom in geology that the land is being lowered 

 at an average rate of about i foot in 4,000 years by meteoric action, 

 by rain and rivers, or all those chemical and mechanical forces that 

 come under the general term subaerial denudation. To this is to be 

 added the mechanical abrasion of coasts. The matter carried into 

 the ocean in solution in river waters is, I have shown on the average 

 of many years, about one-third that in suspension. This seems at 

 first blush a large proportion, but when we consider that the matter 

 in solution is a much more constant quantity than the matter in 

 suspension, our surprise is modified. 



The sedimentary matter — which is chiefly silica either in the 

 form of grains of sand, or in a much finer state of comminution, 

 as flour of rock mixed with the decomposition products of various 

 rocks, notably felspathic, forming what, when deposited, we 

 call clay — is laid down in a more restricted area than the matter in 

 solution. Mixed with these products of denudation are calcareous 

 particles, mica and other minerals, which all go to make up one or 

 other of the various sedimentary strata. These are, as a whole, often 

 looked upon as the effects of mechanical erosion, but, so far as this is 

 an expression of dynamic action, they are only partially so. Chemical 

 forces have, in my opinion, much more to do with loosening the bonds 

 of the rocky particles than has mere pounding of the boulders along 

 shores and river-beds; they also effect the separation of the rock-masses. 

 To be impressed with this fact one has only to look at some of 

 the enormous masses of rock in mountain districts in Wales, moved 

 to their present positions during the last phase of the Ice-age. 

 Although they have been exposed to nothing more than meteorological 

 influences since, they are frequently split up into many separate 

 blocks and are much weathered. Likewise, in granitic districts 

 enormous masses of granitic sand are, as we may say, liberated 

 by the decomposition of granite. 



To these sediments must be added boulders and pebbles which 



go to form conglomerates. These are, however, seldom delivered 



into the ocean by large rivers. They remain in the higher reaches 



or mountain tributaries, so that the boulder-beds found in the sea 



are either the products of coast erosion, the dynamic undermining 



of cliffs by wave action assisted by meteorological influences, or 



are formed by mountain torrents with swift gradients, or are carried 



by glacial agency or by floating ice. 



u 2 



