Life Progressive. 405 



of its predecessor in time. Though it is apparent that there 

 has not been continuity in any given area, still the geological 

 chain could not have been snapped at one point and taken 

 up again at a totally different one. Hence we arrive at the 

 conviction that in geology, as in other sciences, continuity is 

 the fundamental law, and that the lines of demarcation 

 between the great formations are but gaps in our own 

 knowledge. 



Through the study of fossils, as is well known, geologists 

 have been led to the all-important generalization that the 

 vast series of fossiliferous or sedimentary rocks may be 

 separated into a number of definite groups or formations, 

 each of which being characterized by its own organic 

 remains, but not properly and strictly, it must be understood, 

 by the occurrence therein of any one particular fossil. 

 However, a formation may contain some particular fossil or 

 fossils not occurring outside of that formation, thus enabling 

 an observer to identify a given group with tolerable certainty; 

 or, as very often happens, some particular stratum or sub- 

 group of a series, may contain peculiar fossils, whereby its 

 existence may be determined with considerable readiness in 

 divers localities. Each great formation, let it be said, is 

 properly characterized by the association of certain fossils, 

 the predominance of certain families or orders, or by an 

 assemblage of fossil remains that represent the life of the 

 period during which the formation was deposited. 



Fossils, then, not only enable us to determine the age of 

 the deposits in which they are found, but they also further 

 enable us to arrive at some very important conclusions 

 respecting the manner in which the fossiliferous bed was 

 deposited, and, consequently, to the condition of the par- 

 ticular region occupied by the bed at the period of its 

 formation. Beds that contain the remains of animals, such 

 as now inhabit rivers, we know to be fluviatile in their origin, 

 and that at one time they must have either constituted 

 actual river-beds, or been deposited by the overflowing of 



