I 



4 



330 Sketch of the Geology oj 



The concretions called claystones are common at the 

 slide; and I have rarely met with so many of a spherical 

 form. In some instances the fossil shells that have been 

 descrihed are wholly, or in part^ imbedded in these con- 

 cretions. The figure below exhibits the smallest species 

 of Mya^ that has been described, thus partially enveloped. 



£ 



J 



^ 



It would be desirable that some geologist should direct 

 his attention to an elucidation of the subject of these con- 

 cretions. The mode and laws of their production are 

 certamly very obscure; although apparently connected 

 with crystallization. I find an almost universal impres- 

 sion that their forms have been the result of running 

 water; an opinion most clearly erroneous. Some of 

 them have a nucleus at their centre ; and frequently, 

 some of the concentric layers are of different color from 

 the general mass, so that they appear as if the work of 

 art. Indeed, if I mistake not, one of them has been de- 

 scribed within a year or two in Boston, (I have forgotten 

 in what work,) with a drawing, as a curious relic of 

 antiquity. But I have seen the specimen, and am sure it 

 is all a deception. 



^ I have marked on the map only three patches of ter- 

 tiary strata; one a short distance west of Portland, 

 another along Presumpscut river, and a third near the 



occur 



