345 



those stony plates of the same nature which are always to 

 be found associated with them. It is also not an unusual 

 circumstance for many of them to be marked with circu- 

 lar depressions, when, after a short exposure to the at- 

 mospheric influences, they readily disunite, the central 

 portion falling out, carrying with it the woody nucleus 

 that they contained, and leaving the concretions in that 

 regular ring-like form that they so commonly assume. In 

 some instances, while undergoing the necessary process of 

 induration, the particles appear to have shrunk from the 

 centre to the circumference, causing those radiating fis- 

 sures which afterwards became filled by segregation with 

 calcareous spar ; in this case, these concretions become 

 perfect septaria. 



When these carbonized waters flow through the pul- 

 verulent sands which separate the layers of clay, and free 

 from the influence of the roots of neighboring plants, they 

 continue for several feet in length to form those indura- 

 ted plates of concretion that are so frequently to be met 

 with, strewed all over the surface of the ground, far be- 

 neath the level of their original deposition. 



The lower, or amorphous portion of this formation, not 

 being divided into distinct stratification, nor ever having 

 been penetrated by the roots of plants, it consequently 

 would rarely if ever contain these concretionary forms, 

 which seems to be the case ; and from the preceding de- 

 scription it will be seen that although perfectly embraced 

 in the layers of the clay to some considerable depth, they 

 may with strict propriety be considered as beloaging to a 

 more recent period, having been constructed long after 

 the deposition had become complete, and vegetation had 

 for some time flourished in the soil upon its surface. 



Minerals^ in a distinct form, are of rare occurrence in 

 this formation. Besides sulphuret of iron in nodular 

 masses with central radiations, — which are not unfrequent, 

 —some large and beautiful crystals of selenite, or sulphate 



