AN ESTIMATE OF THE GEOl.OGICAL AGE OF THE EARTH. 275 



The g-Iauconite deposits of the deep-sea boundaries and the stratified 

 marine sediments undoubtedl}^ mvist be chiefly made responsible. The 

 composition of this substance is given in the report on the deep-sea 

 deposits of the Ghillenger expedition, where it is shown that it con- 

 tains from 2.52 to 4.21 per cent of potash derived from the sea water. 

 It may amount to 50 per cent or 60 per cent of the shallower deposits, 

 or even more. The percentage of soda is from one-third to one- 

 seventeenth of the potash, and therefore will hardly enter into consid- 

 eration in this paper. 



The formation of this substance appears dependent on the condition 

 that the organic matter in the chambers of foraminifera should reach 

 the bottom, which, if so, will perhaps account for the absence of this 

 body from the deeper deposits. The organic matter ''transforms the 

 iron in the mud into sulphide, which may be oxidized into hj^drate, 

 sulphur being at the same time liberated. This sulphur would become 

 oxidized into sulphuric acid, which would decompose the fine clay" 

 (terrigenous debris), "setting free colloid silica, alumina being removed 

 in solution. Thus we have colloid silica and hydrated oxide of iron in 

 a condition most suitable for their combination." "There is always a 

 tendenc}' for potash to accumulate in the hydrated silicate formed in 

 this way, and, as we have stated before, this potash nuist have been 

 derived from the sea water." ' 



The following extract (p. 384) will serve to show the opinions of the 

 authors (Sir J. Murra}- and Professor Reuard) on the widespread nature 

 of this deposit: 



"It has already been stated that glauconite is one of the minerals 

 most widely distributed in sedimentary rocks. It is found in the pri- 

 mary formations of Russia and Sweden among sands and gravels, in 

 the Cambrian sandstone of North America, in the Quebec Group of 

 Canada, and in the coarse Silurian sands of Bohemia. In the second- 

 ary formations its presence is more pronounced, for example, in the 

 Lias, and especially in the middle and upper layers of the Jurassic 

 system in Russia, in Franconia, in Swabia, and in England. It has a 

 still greater development in the sands, marls, and chalks of the Cre- 

 taceous formation. It will suffice to recall the glauconitic rocks of the 

 Neocomian, of the Gault, and of the Cenomanian in various regions, 

 such as the glauconitic marls of France, Germany, England, and sev- 

 eral parts of North America. The abundance of glauconite is continued 

 into the Tertiary formations, from the lowest up to the highest hori- 

 zons of the series. 



"From this rapid enumeration it is seen that glauconite traverses 

 the whole of the geological periods and its formation is continued in 

 modern deposits along many continental shores explored hy the Ciial- 

 lenger and other expeditions." 



The analyses show that the mineral may vary in composition. 



"All that can be said is that the glauconite now forming at the bot- 

 tom of the sea is, like the glauconite of geological formations, a 



1 Report, p. 389, 



