204 THE JOURNAL OF GEOLOGY. 



for the thickness of the stratified rocks (177,200 ft.), the time required for 

 their formation, he finds to be 28,000,000 years. Mr. Upham next assumes 

 the thickness of the stratified rocks to be 50 miles, and the rate of land 

 denudation to be one foot in 6000 years. This requires 84,000,000 years for 

 their deposition. Estimates of the relative length of geological time 

 divisions are taken from Dana, Winchell and Davis. Estimating the time 

 since the glacial epoch to be 8000 years,, the writer concludes from 'Davis's 

 ratios, that from 16,000,000 to 40,000,000 years have passed since life first 

 appeared on the globe. From changes in the floras and faunas since the 

 beginning of the tertiary, Mr. Upham thinks the length of Cenozoic time 

 is about 3,000,000 years. Applying this to Dana's and Winchell's ratios, he 

 concludes that about 48,000,000 years have passed since the beginning of 

 Cambrian time. " But," says Mr. Upham, " the diversified types of animal 

 life in the earliest Cambrian faunas surely imply a long antecedent time for 

 their development, on the assumption that the Creator worked before then as 

 during the subsequent ages in the evolution of all living creatures. Accord- 

 ing to these ratios, therefore, the time needed for the deposition of the earth's 

 stratified rocks and the unfolding of its plant and animal life must be about a 

 hundred million years." J. A. B. 



Continental Problems. Annual Address by the President, G. K. 

 Gilbert. (Bulletin of the Geological Society of America. Vol. 

 4, pp. 179-190). 



Two-fifths of the earth's area has a mean altitude of- 14,000 feet, the 

 plateau of the deep sea; one-fourth the continental plateau a mean altitude 

 of +1,000 feet; the remaining third includes the intermediate slopes, the areas 

 of extreme depth and areas of extreme height. With the exception of the 

 Antarctic continent, the continental plateau is a continuous area, whereas the 

 plateau of the deep sea is " separated by tracts of moderate depth into three 

 great bodies, coinciding approximately with the Pacific, Atlantic and Indian 

 oceans." The author discusses briefly several of the unsolved continental 

 problems, (i) By some it is suggested that the continental form is maintained 

 by the solidity and consequent rigidity of the earth; by others the materials 

 underlying the continental plateau are supposed to be lighter than those 

 beneath the oceanic plateau. The difference in density is the complement of 

 the diiference in volume. In the author's view the weight of opinion and 

 the weight of evidence is with the latter hypothesis (the doctrine of isostacy)'. 

 Accepting this doctrine, the question is (2) whether the difference in density 

 is due to difference in temperature or difference in composition. To this 

 question no answer can be given at present. (3) For the origin of the conti- 

 nents the author mentions Dana's hypothesis that the continental areas 

 cooled first and the oceanic last. Only negative results were obtained by the 



