160 THE FORMATION OF FLINTS 



bring home to a general audience the great antiquity of 

 the earth, but the danger of such isolated instances lies 

 in the vagueness of the impression they produce. In 

 Huxley's time no doubt they had their use ; at the 

 present day we are convinced believers in the untold 

 aeons of geological time ; but nevertheless we are anxious 

 to take a step in advance, to count these aeons, and to 

 formulate a consistent scheme of geological dates. We 

 have already seen that a by no means niggardly estimate 

 gives a little over 26 million years as the whole period 

 required for the deposition of the stratified rocks of the 

 earth's crust (p. 24). In the table on page 14 the 

 maximum thickness of the Cretaceous system is given 

 as 14,000 feet. Of this, 7,500 feet may be assigned 

 to the upper cretaceous, and at least 5,000 to the chalk. 

 The beds which attain this thickness are chiefly sand- 

 stones, to which our estimate of one foot in a hundred 

 years may apply, so that their equivalent of 1,000 feet of 

 chalk would have consumed half a million years in its 

 formation, and this gives an allowance of forty years for 

 each inch, or a period forty times greater than that 

 demanded by Huxley ample, and more than ample, for 

 the accomplishment of the processes which he describes, 

 and yet perfectly consistent with the conclusions we have 

 reached as to the age of the earth. 



If we are in the mood to push our inquiry as far as 

 possible, we may next demand whence does the sea 

 obtain its store of silica ? 



The sea obtains it in tribute from the rivers, the rivers 

 from springs and the superficial drainage of the soil. 

 It is instructive to observe how this constituent increases 

 in quantity as it is traced towards its source ; thus in the 

 sea, as we have mentioned previously, there is on an 

 average about fourteen parts of silica in a million of 

 water ; in rivers the amount rises to sixty-six in a 



