116 Canadian Record of Science. 



able, that strictly synchronous beds may be found in each 

 of the three zones, though the thickness may vary con- 

 siderably and the strata of some beds may not have 

 synchronous representatives in the other beds. 



During the time of the formation of any given bed (or 

 group of beds formed within a given small interval) the 

 forms of- life existing at that time will be distributed over 

 the surface of that bed, each in its appropriate locality, 

 the sand-loving forms in the sand zone, the mud-loving 

 forms in the areas which afterwards become shale, and the 

 forms which thrive best in deeper or clearer water, 

 beyond. At the transition zones where there is a merging 

 of conditions there will be a merging of forms. It may 

 happen also that a form normally a habitant of one zone 

 is accidently carried to another. These may be regarded 

 as accidents in the normal distribution. Occasionally a 

 form migrating from one zone to another will, in the pro- 

 cess of its migration, take on different features because of 

 its environment, eventually undergoing such changes that 

 it would be classed as a species different from the 

 descendants of its parent forms which remained in the 

 original habitat. Some few forms may exist in all three 

 zones, and fossil forms of these types will be of special 

 value in the correlation of beds of different zones. It must 

 be noted, however, that in the study of fossil forms which 

 are fonnd common to all three zones the question whether 

 the forms have migrated during the course of time from 

 one zone to another must be considered, and the beds in 

 which such similar forms are found need not necessarily 

 be of the same age, but may be successively older or 

 younger than the bed in which the like forms are found. 



In the production of a series of deposits the final pro- 

 duct is the function of many varying factors. The two 

 most important of these factors, and which for this reason 

 may be termed primary factors, are the Rate of Depres- 

 sion of the. Land and the Rate of Supply of Detritus. 

 Such a factor as the Character of the Material may, by 



