ANALYTICAL ABSTRACTS. 205 



author in examining the configuration of the continental mass in order to see 

 whether it might belt the earth in a great circle. (4) The causes of differential 

 elevation and subsidence within the area of the continental plateau is yet 

 unknown, but in the opinion of American geologists these differential 

 changes of level are conclusive proof that the changes are in the lithosphere 

 and not in the hydrosphere. (5) The doctrine of the permanence of conti- 

 nents is regarded as not yet fully established. (,6) The growth of the 

 continents, also, is considered as a question still open to discussion. The 

 author does not think it is fully proved that continental growth has been as 

 steady a process as is generally believed. Most of the evidence appealed to, 

 and the inferences drawn therefrom, concern only the minima of ancient land. 

 The data of unconformities, by which the maxima can alone be determined, 

 are comparatively few, are usually dil^cult of determination, and therefore 

 have never been fully assembled. Further search ought to be made along 

 these lines before this question can be considered closed. 



H. B. K. 



Measurement of Geological Time. By T. Mellard Reade, C.E., 

 F.G.S. (The Geological Magazine, March, 1S93). 

 The particles of which the sedimentary rocks are composed have been 

 used again and again in rock building, but were all originally derived from 

 the pre-Cambrian rocks or from igneous rocks of later date. By estimating 

 then the average area of the pre-Cambrian and igneous rocks, the bulk of 

 sedimentary rocks derived from them during Cambrian and post-Cambrian 

 times, and the rate of erosion, calculation may be made of geologic time since 

 the beginning of the Cambrian. The author assumes " for the sake of the 

 calculation," the average area of the pre-Cambrian and igneous rocks to be 

 one-third the whole land area of the globe. The actual bulk of the sedi- 

 ments accumulated since the beginning of the Cambrian is estimated as equal 

 to the present land area two miles thick. The average rate of erosion is 

 taken as one foot in 3000 years. From these estimates the time that has 

 elapsed from the beginning of the Cambrian is in round numbers 95 millions 

 of years. When the enormous length of pre-Cambrian time is added to the 

 above, the estimate is found to agree very closely with that of Sir Archibald 

 Geikie, i.e., 100 to 600 millions years. H. B. K 



Recent Archcsological Explorations in the Valley of the Delaware. By 

 Chas. C. Abbott, M.D. (Publications of the University of 

 Pennsylvania). 



This work gives the results of the author's recent investigations in the 

 Delaware Valley, principally at two islands near the head of tide water. 



