Correspondence — The Crust of the Earth. 287 



aerial with that of marine denudation. Before positive estimates 

 were made in regard to the two rates, no one ever imagined that 

 marine denudation was so trifling in comparison to sub-aerial. Is 

 the amount of materials deposited in the submarine volcanoes equal 

 to that derived from marine denudation ? The sea is continually at 

 work, but volcanoes are only now and again in eruption. If the 

 materials supplied by submarine volcanoes be not greater than that 

 by marine denudation, all the three sources which we have been con- 

 sidering put together, must fall far short of supplying an amount of 

 material equal to that supplied by sub-aerial denudation. There is 

 this, however, to be said of volcanoes, viz., the materials which they 

 do produce — lava and trap-rock — resist denudation, and are conse- 

 quently better preserved than rocks formed out of materials derived 

 from sub-aerial and marine denudation. This, no doubt, is the 

 reason why, in rock sections the traps bear so large a proportion to 

 the sandstones, shales, and other softer rocks. Still further, were it 

 not for the protection afforded by cappings of trap, the sedimentary 

 rocks would be much thinner than they actually are. 



Edinbukgh. James Croll. 



THE CRUST OF THE EARTH. 



Sir, — With your permission I should like to ask a question or two 

 suggested by one part of Mr. Porbes's very instructive paper in the 

 April number of your Magazine. 



Is not the comparison there drawn between the crust of the earth 

 and the shell of an egg likely to produce a somewhat inaccurate 

 impression, which it is as well to avoid, and especially so when, as 

 in the present instance, the illustration to some extent does duty as 

 an argument? In the case of an eggshell, the vault consists of a 

 single piece, so that the form is largely aided by the force of co- 

 hesion in supporting a load. The weight is equally distributed by 

 the former, but transverse fracture, crushing of the material and 

 the shearing, or sliding of particle on particle, are, up to a certain 

 extent, prevented by the latter. It is, however, improbable that in 

 the crust of the earth no joints or fissures exist reaching continuously 

 or interruptedly from the top to the bottom. The igneous rocks, as 

 we know them, are so fractured that it is hard to find a mass of 

 many yards in length free from cracks and flaws. Does not the 

 presence of these destroy the analogy between the crust of the earth 

 and an unbroken eggshell ? and every one can see how deft a hand 

 would be required to build up the fragments of a broken one, so that 

 it should bear even its own weight. Is it not more correct to liken 

 the crust of the earth to a heavy, but unloaded arch, whose voussiors 

 are constantly sliding on one another, in consequence of the ever- 

 varying strain thrown upon the different parts, and the necessity 

 of preserving equilibrium ? Considerable up-and-down movement 

 would thus be allowed, evidence of which may be seen in the enor- 

 mous throws sometimes shown by faults. Cavities, too, might exist 

 underneath the vault without endangering its stability, their size 

 being limited by the ability of the material to resist crushing. 



