716 THE JOURNAL OF GEOLOGY. 



alternative but to conclude that the agencies which were con- 

 cerned in its production were capable of wide-spread action. 



Not only do we conclude from the lithological heterogeneity 

 of the drift that it came from wide-spread sources, but we may 

 reach positive conclusions concerning the minimum areal limits 

 of these sources. By a careful study of the lithological char- 

 acter of the stones of the drift at any given point, and by com- 

 paring them with the formations of solid rock in all directions from 

 this point, it is generally possible to determine the exact forma- 

 tion from which many of them came. If the surface exposures 

 of the formations from which the identifiable bowlders came be 

 very small, we are able to draw definite and positive conclusions 

 concerning both the direction of transport of the bowlders in 

 question, and the distance which they have journeyed. If the 

 surface exposures of the formation whence they came be large 

 instead of small, the conclusion which could be drawn would be 

 confined within less narrow limits. 



If the determination of the direction and distance of trans- 

 port rested on the identification of a single type of rock, it would 

 be necessary, in order to make the conclusion absolute, to prove 

 that there is no second source, seen or unseen, whence the given 

 type of bowlders could have come. Manifestly this might be a very 

 difficult thing to do. But if identity can be established between 

 various and diverse types of rock found in loose pieces in the 

 drift of a given locality, and an equal number of formations in 

 beds or in situ elsewhere, the case becomes much stronger, for 

 it is almost beyond belief that two complex but lithologically 

 dentical series of rock formations, embracing bowlders of so many 

 types as frequently occur in the drift, exist in different directions 

 from any given area. It follows that where identity can be estab- 

 lished between a wide-ranging series of bowlders and rock frag- 

 ments in the drift, and an equally complex succession of diverse 

 formations in situ lying in a common direction from the drift 

 which contains the identified types of rock, the conclusion that 

 the former came from the latter is so highly probable as to 

 amount to moral certainty. The known facts concerning the 



