Date of the Glacial and the Upper Miocene Period. 143 



period not exceeding something like 100 millions of years. 

 Taking- the age of the earth's crust at 100 millions of years, 

 which probably is a high estimate, how long is it since the mate- 

 rials which now form these Laurentian rocks were laid down in 

 the ancient sea-bottom in the form of sand and mud ? This is a 

 question which no one can answer; for we have no means of 

 knowing how much of the 100 millions of years were exhausted 

 before the Laurentian age. At all events, it must be very consi- 

 derably less than 100 millions of years since the commencement 

 of the Laurentian period. 



Sir William Logan thinks that the time which separated the 

 limestones containing the Eozoon Canadense from the Upper- 

 Cambrian period may be as great as the time which elapsed be- 

 tween the Upper Cambrian and the nummulitic limestones of the 

 Tertiary period. If this conjecture be anything like correct, 

 then it is hardly possible that 50 millions of years can have 

 elapsed since the Cambrian period. Assuming that the glacial 

 epoch began about a quarter of a million years ago, and that the 

 rate at which species change is uniform, we found that, adopting 

 Sir Charles Lyell's mode of calculation, 60 millions of years have 

 probably elapsed since the beginning of the Cambrian period. 

 But I presume little or no weight can be placed on this mode of 

 calculation ; for it is based upon an assumption for which there 

 is, I fear, very little warrant, viz. that the rate at which species 

 change has been anything like uniform during geological ages. 

 If any very great amount of time elapsed between the solidifying 

 of the earth's crust and the Laurentian period, the probability is 

 that the commencement of the Cambrian period does not date so 

 far back as 60 millions of years. 



Table I. shows the excentricity of the earth's orbit and longi- 

 tude of the perihelion for 3 millions of years back, and 1 million 

 of years to come, at periods fifty thousand years apart. From 

 what we have already seen, it is very evident that 3 millions of 

 years must stretch a considerable distance back into the geolo- 

 gical history of our globe. And if geological climate depends 

 upon, or is much affected by, the condition of the earth's orbit 

 as regards excentricity, we have in this Table, combined with 

 the other three, the means by which a rou2;h idea of the cha- 

 racter of the climate during at least a considerable portion of 

 the Tertiary period may be arrived at. 



