MARYLAND GEOLOGICAL SURVEY 103 



considerable light on the distribution of EUphas and certain other forms 

 in eastern North America. 



SIMILARITY OE MATERIALS. 



Correlations founded on evidence afforded by similarity of materials 

 is always open to question and unless the evidence is used with the great- 

 est caution, it is apt to be more harmful than helpful. This class of 

 criteria has been of some aid. in correlating the various terraces within 

 the Coastal Plain, but on the whole has been found inadequate. The 

 deposits may be divided into two great categories, those which carry 

 large boulders and those which do not. It has been found that the 

 highest terrace deposits, or those which have been assigned to the Lafay- 

 ette formation do not contain these boulders, while the Sunderland, 

 Wicomico, and. Talbot formations are abundantly supplied with them. 

 For a number of reasons it is evident that most of these boulders are ice- 

 borne. If this conclusion is well founded, and if the Lafayette has been 

 correctly referred to the Pliocene period, ice-borne boulders would not be 

 expected in that formation, for it was deposited during a time of genial 

 climate at least in these latitudes. The presence of ice-borne boulders in 

 the Lafayette of Maryland would, therefore, immediately call into ques- 

 tion the Pliocene age of this formation. This means of separating the 

 Lafayette from the other terraces is helpful but not altogether reliable, for 

 blocks of floating ice are not the only means by which boulders can be 

 transported for long distances and deposited in the midst of fine silt. 

 Floating vegetation can also perform this function as well as ice. The 

 mere presence, therefore, of a few boulders in the Lafayette imbedded in 

 fine silt would not necessarily point to ice action. Neither would it be 

 sufficient evidence to correlate the deposit with one of the Columbia for- 

 mations. Vast areas of the Lafayette have not as yet been dissected by 

 streams and are consequently not accessible to the geologist, and it would 

 not be surprising if here and there scattered boulders should intimately 

 be found within it. They have been found occasionally in the Miocene of 

 Maryland in the midst of fine marine silt. Why may they not be ex- 

 pected in the Lafayette formation? On the other hand, the absence of 



