80 



TRANSITION ROCKS. 



CHAPTER VII. 



TRANSITION AND SECONDARY ROCKS. 



Transitbr. Rocks. — How Divided. — Slate. — Hornstone. — Whet- 

 stones. — Hones. — How Slate is Formed. — Its Cleavage. — 

 Transition Limestone.— Graywacke.— Old Red Sandstone.— 



Claystone. 



We now come to consider the next division of 

 rocks, called transition or intermediate , because they 

 lie between the primary and secondary, partaking, 

 in some respect, the character of both. It is in 

 some of the rocks of this class that we begin first 

 to discover the fossil remains of animals and vege- 

 tables, and they may therefore, as Bakewell re- 

 marks, be regarded as the most ancient records of 

 organic existence on our globe. 



The rocks generally described as belonging to 

 this class are the following, viz. : 



1. Slate (Argillaceous). 



2. Transition or Mountain Limestone. 



3. Graywacke. 



1. Slate. 



We have already described slate* as one of the 

 simple minerals. We may farther remark, that it 

 abounds in most hilly and mountainous countries, 

 resting either on granite, gneiss, or mica slate. 

 Where it is found resting immediately on the pri- 

 mary rocks, it has a more shining lustre, and par- 

 takes more of the crystalline quality of mica slate. 

 As it recedes from the primary rocks, it becomes 



* Where the word slate is used alone, we mean by it argilla 

 ceous or clay slate. 



