CHAP. X 



THE EARTH'S AGE 



219 



will form most quickly near the shores, and will thin out 

 rapidly at greater distances, little or none being formed in 

 the depths of the great oceans. This important fact was 

 demonstrated by the specimens of sea-bottom examined 

 during the voyage of the Challenger, all the " shore 

 deposits " being usually confined within a distance of 100 

 or 150 miles from the coast ; while the " deep-sea deposits " 

 are either purely organic, being formed of the calcareous or 

 siliceous skeletons of globigerinse, radiolarians, and 

 diatomacese, or are clays formed of undissolved portions of 

 these, together with the disintegrated or dissolved 

 materials of pumice and volcanic dust, which being very 

 light are carried by wind or by water over the widest 

 oceans. 



From the preceding considerations we shall be better 

 able to appreciate the calculations as to the thickness of 

 stratified deposits made by geologists. Professor Ramsay 

 has calculated that the sedimentary rocks of Britain alone 

 have a total maximum thickness of 72,600 feet; while 

 Professor Haughton, from a survey of the whole world, 

 estimates the maximum thickness of the known stratified 

 rocks at 177,200 feet. Now these maximum thicknesses of 

 each deposit will have been produced only where the con- 

 ditions were exceptionally favourable, either in deep water 

 near the mouths of great rivers, or in inland seas, or in 

 places to which the drainage of extensive countries was 

 conveyed by ocean currents ; and this great thickness will 

 necessarily be accompanied by a corresponding thinness, or 

 complete absence of deposit, elsewhere. How far the 

 series of rocks found in any extensive area, as Europe or 

 North America, represents the whole series of deposits 

 which have been made there we cannot tell ; but there is 

 no reason to think that it is a very inadequate representa- 

 tion of their maximum thickness, though it undoubtedly is 

 of their extent and hulh. When we see in how many 

 distinct localities patches of the same formation occur, it 

 seems improbable that the whole of the deposits formed 

 during any one period should have been destroyed, even in 

 such an area as Europe, while it is still more improbable 

 that they should be so destroyed over the whole world ; and 



