248 



CHAP. XXXVIL 



Shale, Clay, Sand, Marl. 



THE term Shale includes all the secondary and ter- 

 tiary argillaceous schists ; but I do not notice its pro- 

 vincial names, while ihe ill-judged term slate-clay is, 

 rejected, as unnecessary and inconvenient. 



It resembles the primary schists so much, in mineral 

 characters, that they cannot be distinguished by any 

 general definition ; and though some of its varieties 

 may differ from every one in the latter division, there 

 are many which are undistinguishable. As in other 

 cases, the distinctive character of this rock is com- 

 pounded of its geological associations and its mineral 

 nature ; and it is only through both that it can safely 

 and effectually be recognised. Its varieties are enu- 

 merated in the Classification of Rocks ; and I need 

 only remark here, that it is more or less indurated, and 

 that its prevailing colours range from grey to black, 

 but comprise also red and yellow hues of considerable 

 variety. When highly ferruginous, it passes into argil- 

 laceous ironstone ; and, becoming calcareous, into 

 schistose marl, and into limestone. 



In this case in particular, or when inter^t ratified 

 with limestone, it is the frequent seat of fossil shells ; 

 while the argillaceous beds of such a series often con- 

 tain a greater proportion of these than the calcareous 

 rock. In the coal series, it often contains bitumen, 

 orcarbonaceous matter, or both ; and, in the same 

 circumstances, is the frequent repository of vegetable 

 remains. The most bituminous varieties, known by 

 the name of Kimrneridge coal, are combustible. 



Otherwise compounded, it contains mica, or quartz 



