".II The American Geologist. December, 1893 
ment. The geologist realizes that geologic time cannot be reduced to 
actual time in decades or centuries; there are too many partially rec- 
ognized or altogether unknown factors; but he can approximate the 
relative position of certain formations and by comparison of their sedi- 
ments, dimensions and contained recoidof life with estimated rates of 
denudation, sedimentation, and organic growth, form a general estimate 
of their relative time duration. It is my purpose to-day to take up the 
consideration of the evidence afforded by the sedimentary rocks of our 
continental area and largely of a distinct basin of sedimentation with 
a view of arriving, if possible, at an approximate time period for their 
deposition. Before so doing. I will briefly refer to a few of the opinions 
that have been held by geologists on geologic time and the age of the 
earth. Soon after geology emerged from itspre-systematic stage, in the 
latter part of the eighteenth century, and assumed an independent po- 
sition among the inductive sciences, speculations on the age of the 
earth began. Dr. James Hutton, the founder of modern physical 
geology and the predecessor of Lyell, in advocating the uniformitarian 
theory, was the first to argue that the rate of destruction of one land 
area was the means of measuring the duration of others, and that the 
continents were formed of the ruins of pre-existing continents, but that 
in our measurement of time such periods were of indefinite duration.* 
It was not, however, until 1830, when Sir Charles Lyell published 
the results of his profound and philosophical studies of geologic phe- 
nomena, that the broad outlines of the law of uniformity, as opposed to 
the doctrine of geologic catastrophes, was firmly established. This work 
rendered possible a computation of the age of the earth on the princi- 
ple that geologic processes were the same in the past as at present. He 
based his estimate of time on a rate of modification of species of mol- 
lusca since the beginning of the "Cambrian period," and divided the ge- 
ologic series into twelve periods, assigning 20,000,000 years to each for a 
complete change in their species, — or 240,000,000 years in all. This esti- 
mate excluded the "antecedent Laurentian formation.'"! 
The hour at our disposal does not permit of mentioning at length the 
views of other geologists. Dr. Charles Darwin thought that two 
hundred millions of years could hardly be considered sufficient for the 
evolution of organic formo,J and Rev. Samuel Haughton assigned 1280 
millions of years to pre-Azoic time, and remarked that the globe was 
habitable, in part at least, for a longer period§. At a later date he es- 
timated a minor limit to geologic time of two hundred millions of 
years.; Dr. James Croll estimated 72 millions years for the time dura- 
tion since the first deposition of sedimentary rocks, while Sir Alfred 
R. Wallace thouerht that 28 millions would suffice.** Of the value of 
*Theory of the Earth; or an Investigation of the Laws observable in the Com- 
position, Dissolution and Restoration of Land upon the Globe. Trans. Royal 8oc. 
Edinburgh, vol i, 17S8, pt. 1, p. 304. 
fPrinciples of Geology, 10th Ed., vol.1, i867, p. 301. 
^Origin of Species, American Ed., from 6th Kng. Ed., 1882, i>. 286. 
glianoal of Geology, 3rd Ed., 1871. p. 101. 
Nature, vol. 18, 1878, pp. L'67-2ti8. 
**Stellar evolution and its relations to geological time, 1889, pp. 48- 1!'. 
