400 The Parallel Eoads of Glen Boy. 



we think that his answer to the objections is upon the whole 

 satisfactory. 



The absence of marine remains is accounted for by the ten- 

 dency of such annual substances to decay. On the coast of 

 Forfarshire Mr Lyell found sea shells in gravel beds extend- 

 ing to the height of fifty or sixty feet, but in similar deposits 

 of gravel, at greater altitudes, he found none, doubtless be- 

 cause the higher beds being first out of the water, and longest 

 exposed, the organic remains in them had been decomposed. 

 In Norway, near Christiana, there ai*e stratified deposits of 

 clay and sand ascending to the height of 600 feet above the 

 sea ; but shells are only found in those patches which are un- 

 der 200 feet, and thej' are rare, and generally much decom- 

 posed, in those whose height exceeds 50 feet. 



Mr Darwin, after citing these and other similar facts, adds, 

 " In the extensive and superficial beds of elevated shells on 

 the coast of Peru, where rain does not fall, and where conse- 

 quently loose matter is not washed from the surface, I have 

 traced, as I have ascended from the beach, a most perfect gra- 

 dation in the decay of the shells, until a mere layer of calca- 

 reous powder without a vestige of structiure alone remained."' 



Sweden is rising from the sea at the rate of three feet in a 

 centiu-y. If we suppose the elevation of the soil of Scotland 

 to have taken place at the same rate, the 900 feet of differ- 

 ence of level, between the Forfarshire beds which contain 

 shells, and the lowest terrace in Glen Roy which contains 

 none, will correspond to a period of 30,000 years ; and from 

 the facts just cited, we may safely infer, that a much shorter 

 exposure would suffice to obliterate every trace of organic re- 

 mains fi'om the terraces, supposing that they originally existed 

 there. 



As to the second objection, the non-continuity or entire ab- 

 sence of the shelves, Mr Darwin's argument involves many 

 details, and we can do little more than allude to it. He 

 shews that a combination of circumstances, perhaps rarely oc- 

 curring, was necessary to produce them. Among these are, 

 a considerable inclination of the surface, the absence of bare 

 rock, the existence of moveable debris above, the non-exist- 

 ence of strong currents. Add to this, that rains and storms 

 must have had a considerable effect in obliterating the 



