LOWEST RED SANDSTONE. 395 



It has often been the practice with geological 

 writers, as has already been remarked, to com- 

 prise under the name, not only of this rock, but 

 of many others, several substances of a distinct 

 mineral character, which are, either frequently, 

 or usually, associated with the principal rock. 

 These are generally, if not always, in smaller 

 quantity than the leading substance which gives 

 a name to the whole ; and are found, most com- 

 monly, in repeated alternation with it. 



The term formation has been applied to such 

 groups of alternating strata, and the associated 

 substances are considered as subsidiary, or subor- 

 dinate to the leading rock of the whole. It is 

 not here the place to enquire respecting the uti- 

 lity or the propriety of this practice, or of that of 

 the terms above mentioned : it is a question that 

 appertains to geology. But in a mineralogical 

 description of rocks, the use of a term so general 

 as that of formation? thus applied, would evi- 

 dently be improper ; as, under the red sandstone 

 for example, in the instance now under examin- 

 ation, there would be found, not only shales of 

 various kinds, but limestone. These are there- 



