102 0. D. Hubbard — Colloids in Geologic Problems. 



These salts are all three in the sea. The sodium chlo- 

 ride makes up fully three-fourths of all the salts there, 

 while the alum constitutes a very small fraction of one 

 per cent, but because of its greater activity, it may be 

 as efficient as the much more abundant sodium chloride. 

 For our purposes, however, it is not material which does 

 the work. A much more important question is, how long 

 have the seas been salt enough to coagulate the fine clays 

 in colloidal suspension as they come down? "We have 

 shale rocks made of such clays of all ages back through 

 the Mesozoic, Paleozoic, and perhaps well into the Proter- 

 ozoic. Such clays of course would settle out of quiet 

 waters without the assistance of an electrolyte, but such 

 waters certainly would be difficult to find on the conti- 

 nental shelf. Therefore we may safely assume that these 

 shales made of extremely fine clays were largely precip- 

 itated by electrolytes, and that the sea was salt enough in 

 those Proterozoic days to effect their coagulation. 



Experiments show that one percent, sodium chloride 

 solution will coagulate clays rather quickly and that one- 

 tenth per-cent. alum solutions will bring them down in a 

 few hours, so we need not assume that the sea had a 

 greater saltiness than, say, 25 per cent, of its present 

 salinity in order to bring down the clays. In this connec- 

 tion it may be interesting to note that the age of the 

 ocean has been calculated on the basis of the NaCl in the 

 water, assuming essentially the same rate of inflow as 

 at present, and only trifling losses as compared with the 

 present salt in the sea. The latter assumption is easily 

 established. The former is probably safe. Dividing the 

 rate by the amount of salt in the sea, Chamberlin and 

 Salisbury arrive at the figure of 370,000,000 years. If 

 anything like one-fourth the present saltiness would be 

 necessary before effective precipitation should take place, 

 then these old shales may be at least 250,000,000 years old, 

 a much larger figure than usually given by geologists for 

 the period since early Proterozoic. On the basis of these 

 figures they may not be more than 300,000,000 years old. 

 Pirsson and Schuchert's geology gives 80-90 instead of 

 370 million years. 



Many colloidal suspensions, like those in the muddy 

 waters of lakes, owe part of their stability to the fact 

 that their particles all carry like charges. Of course, the 



