MEASURING GEOLOGIC TIME— LANE 245 



ing salt water, the sodium absorbed by sediments in base exchange, 

 and for the rest of the factors tending to increase his estimates of time 

 and those tending to diminish them which Joly Ksted, we can estimate 

 how long it would take at the present rate to accumulate the salt at 

 present in the ocean. This Joly first did, and his work, first published 

 in the Transactions of the Royal Society of Dublin, has already 

 received attention in the Smithsonian Report.^® F. W. Clarke has 

 gone over the figures with ampler data.^" 



Jones has estimated the time required to make the Lakes Truckee 

 and Winnemucca, shrunken remnants of the large lake Imown to 

 geologists as Lake Lahontan, as salt as they are in three ways: First, 

 by comparing the increasing saltness given by two analyses 30 years 

 apart; second, by studying the rate of evaporation and the time needed 

 to evaporate the water; and, third, by taking the analyses of the rivers 

 feeding these lakes and computing the time requu-ed to bring in the 

 salt.2» 



Of these only the latter method has been used to obtain the age of 

 the ocean, and even in that there are difficulties which prevent our 

 relying upon the figures. In the first place, the analyses of river 

 waters upon which estimates of the yearly contribution of sodium 

 were based do not give enough weight to the composition in time of 

 floods, at which most of the run-off takes place. In the second place, 

 there is no assurance that the present climate and run-off is a fair 

 average of that in past times. In fact, many geologists believe that a 

 period of extra uplift and glaciation, exposing a lot of rock to weather- 

 ing processes, has occurred in the immediate past, and that the present 

 state of things is not yet back to average. In the third place, if we 

 may trust Schuchert's paleographic maps, the present continents are 

 in general larger than in the past. 



All these factors would tend to make the estimate of time of accumu- 

 lation of oceanic sodium (about 100 milHon years) too low. On the 

 other hand, any original sodium in the ocean water, from whatever 

 source, would make the estimate too high. 



Moreover, if we compare any other element than sodium in river 

 water and the ocean, we get very different results. Some of these 

 elements, such as calcium and magnesium, are promptly seized by 

 organisms to make shells. But chlorine offers a different story, and 

 there is reason to look for a volcanic source for much of the chlorine 

 and to suppose that it was more abundant in the early ocean. Indeed, 

 endeavors have been made ^^ to estimate geologic time from the change 



>• Joly, J. Smithsonian Ann. Rep. 1899, pp. 247-288, 1901. 



'» Clarke, F. W. Data of geochemistry. U. S. Geol. Surv. (5 editions), especially chs. 1-5, and pp. 

 146-151. 



" Jones, J. C, Geological history of Lake Lahontan. Carnegie Inst. Washington, Publ. 352, pp. 1-50 

 1926. 



» Lane, A. O., Geological column; France, Britain, Germany, United States. Lefax, pp. &-357, October 

 1919, Philadelphia. 



