Page:The Journal of geology (IA journalofgeology11893univ).pdf/221

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 continents 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 difficult 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.

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 sediments 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.

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.