322 I. Boivman — Atlantic Preglacial Deposits. 



the varying physical conditions under which sedimentation 

 takes place in shallow waters. Controlled by the evidence 

 of the unconformities it has, however, a more certain value. 

 The relative thinness of the deposits close to this, their 

 northern limit, and the frequent unconformities point to 

 even greater irregularity in the physical conditions here than 

 farther south, the deposits being more readily affected by 

 slight oscillations of level common to the lands. The com- 

 plexity of the mutual relations of coastal deposits is enhanced 

 by the fact that continued erosion and redeposition often results 

 in the close lithologic resemblance of beds of quite different 

 age. In such cases there is, therefore, small value in the con- 

 clusions based on evidence of this sort. Unconformities and 

 fossil evidence are the closest available determinants under such 

 circumstances, and it is quite largely on the former that the 

 conclusions of this paper are based. 



The nearest known outcrop of beds lithologically similar to 

 those at Third Cliff is at Gay Head, Martha's Vineyard. These 

 are described by Professor Woodworth,* who mentions the 

 following characters of the successive beds : 



Lower Cretaceous : non-marine, lignitic, leaf-bearing clays. 



Upper Cretaceous : locally hardened bands of sands con- 

 taining molds of fossils, locally developed beds similar to those 

 at Indian Hill (M. V.) which have a texture varying from tine 

 to coarse with scattered larger grains of quartz and abundant 

 muscovite scales. Inferred unconformity between Lower and 

 Upper Cretaceous. 



Miocene : thickness varies from to 10 feet. Consists of 

 two members — osseous conglomerate and foraminiferal or 

 greensand beds, with unconformity between. The former is 

 from 12 to 18 inches in thickness, consists of rounded bowlders 

 or of nut-sized quartz pebbles white and well-rounded. Ceta- 

 cean bones present. The foraminiferal bed is from to 10 

 feet thick, green color below, brown above, basal part includes 

 rolled fragments of osseous conglomerate and bears glauconite 

 casts of Macoma lyelli in the attitude of growth and the crab 

 Archeoplox signifera in lower part of stratum. 



Probable Pliocene: yellowish green and brownish clays 

 bearing glauconite and Pliocene fossils. Inferred unconform- 

 ity between Pliocene and Miocene. 



The paucity of glauconite in the Gay Head Upper Creta- 

 ceous and the variable texture of the material, the scattered 

 quartz grains, abundant muscovite scales, correspond precisely 

 witli the conditions found in the white and yellow sands which 

 form the basal member of the Third Cliff section. Even the 

 occurrence of cross-bedding on a small scale in the coarse pre- 



* "Unconformities of Martha's Vineyard and Block Island" ; B. G. S. A., 

 viii, 197-212, 1897. 



