266 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



ered in the estimate of river suppl}^ to the ocean furnished bj^ Sir J. 

 Murray, which takes into account only that falling directly into the 

 ocean. The drainage of the "rainless'' regions of the earth— regions 

 where the rainfall is less usually than 11 inches per annum, and which 

 do not drain into the sea — is excluded. As in the present, so in the 

 past, we conchide that such regions existed scattered over the land 

 surface at various periods of the earth's history; and we tind no ])et- 

 ter confirmation of the preservation of the present climatic conditions 

 than exists in these telltale beds of saline deposits. They furnish a 

 striking support to the uniformitarian views here advocated. 



At the best the stratified salt deposits of the earth must form only 

 a v^ery small fraction of what has accunudated in the waters of the 

 ocean. The rock salt of the latter would cover the entire dry land of 

 the earth to a depth of 400 feet. The other deposits are entirely local, 

 and but rarel}' attain this thic-kness. We see from what has I)een said 

 that the fractional part of some of these deposits, which actually go to 

 throw error into our calculations, makes so small a part of a small 

 correction that we are not concerned with its estimation. 



\. — THE ALKALIES OF THE KOCKS. 



It is a fact of great interest in connection with our present consid- 

 eration that the igneous and eruptive rocks, as a whole, possess an 

 amount of soda alkali which preponderates over potash alkali, while 

 in the case of the sedimentary I'ocks this is in the very large niuuber 

 of cases reversed, the potash alkali exceeding the soda alkali. 



This becomes clear in the light of what we have already considered 

 as regards the gradual derivation of the salts of the ocean in the 

 process of formation of the sedimentaries, coupled with the fact that, 

 under or during conditions of weathering, potash aluminium silicates 

 are more resistant than soda aluminium silicates. This and another 

 cause for the retention of the latter salts will be reverted to again. 

 The fact we wish to dwell on here is the ultimate one that this chem- 

 ical distinction, broadly speaking, exists between the igneous and 

 sedimentary rocks. We shall also find that the restoration of the 

 known amount of sodium in the ocean to the sedimentary rocks will 

 bring them up to the sodium percentage of the igneous rocks. A like 

 restoration can not be eifected for the potash alkali, owing to reasons 

 we have briefly to point out further on.^ 



The average igneous and eruptive primitive crust rock arrived at 

 by Mr, Clarke {a/itf) possesses the alkali percentage: 



K2O . - . . : 2. 83 



Nap 3.61 



^In dealing with this, question in this and. the ensuing section the sodium and potas- 

 sium of the sea will be calculated as the oxides, for the convenience of frequent ref- 

 erences to rock analyses. 



