80 
TRANSITION ROCKS. 
CHAPTER VII. 
TRANSITION AND SECONDARY ROCKS. 
Transition Rocks.— How Divided. — Slate. — Hornstone.— Whet- 
stones.— Hones.— How Slate is Forme^.— 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. 
