ON BIOLOGY. 37 



rocks; the Devonian age, represented by the series of 

 rocks in which the ichthyian or fish forms culminated; 

 the age of Amphibians, represented by the rocks of the 

 carboniferous strata; the age of Reptiles, represented by 

 the rocks of the Secondary system; the Mammalian age, 

 represented by the Tertiary and Quarternary; and lastly, 

 the age of Man, represented by the recent rocks. 



Unconformity in the rock system, though of a less 

 general nature, also furnishes the necessary basis for the 

 subdivisions of eras and ages into the minor subdivisions 

 of periods and epochs. So much, then, for the methods 

 adopted by geologists for demonstrating the natural 

 divisions of the earth's crust and the names they have 

 applied to those divisions. The subject includes a very 

 small part of the science of geology one chapter out of 

 a great many, many others but it will suffice our pur- 

 pose here. It will hardly be necessary to add that the 

 Laurentian rock system of the Archaean era is the oldest 

 known to geologists, the others in the order we have pre- 

 sented them being successively more and more recent 

 until we arrive at the Psychozoic or the present horizon. 

 ' It is not to be believed, however, that the very lowest 

 stratum of the Laurentian rocks represents the original 

 or primitive crust of the earth, for there is evidence in 

 them of stratification which abundantly indicates that 

 they have been formed from other rooks which underlie 

 them, and these likely from still others and vastly older 

 ones of which we as yet have no definite knowledge. 

 When one appreciates the manner in which these rocks 

 were built up, and realizes the fact that in some parts of 

 the world they exhibit a thickness of 40,000 feet, it is not 

 difficult to conceive of the enormous lapse of time they 

 represent. As a matter of fact the Archaean era extends 

 over a greater length of time than all of the remaining 

 history of the earth put together. We have all heard, at 

 differ nt times, of the enormous periods of time that 

 geologists insist upon when speaking of the processes 

 that have led to the formation of the crust of the earth 

 as it is now presented to us. It will be quite in point in 

 this place to dwell for a moment upon this matter, as it 

 is of importance in connection with what I have to say 

 further along. There are those who believe, though it is 

 a satisfaction to know that their number is steadily de- 

 creasing, that geologists are prone to indulge in a species 

 of guessing when they come to estimate the lapses of 

 time that have taken place during the various eras, ages, 

 periods, etc., of the world's history, as recorded in the 



