36 CHARACTERS OF SLATE. 
presume the term clay-slate may be applied,is found 
assuming for the most part the appearance of small © 
and loosely joined fragments of a light grey colour, 
sometimes blueish-grey, much tinged with iron, and. 
frequently intersected by seams of quartz, also brown, 
whitish yellow, red, greenish, &c. At intervals, these 
hills of clay-slate give good quarries of roofing ma- 
terial, of a bright lead colour, in which, fossils are 
distributed. These quarries are in general soon 
worked out, the stone soon becoming coarse, or 
degenerating into arubbly state, ora “ grey dunstone” 
which is an indurated form of clay-slate. The course 
and dip of our slate vary greatly, even at times 
within a small space. I have understood, that near 
Exeter is a bed of slate horizontal in its course. 
This, it was thought was a rare circumstance, but 
in this neighbourhood it is far from being an uncom- 
mon appearance, though only observed in small 
patches. From a level, it varies to a close approx- 
imation to verticality. It faces usually from about 
south-west to about south-east, but likewise, at times, 
looks southwardly, eastwardly, and westwardly or 
thereabouts; but, though it is thus prone to variation, 
even repeatedly within a small distance, I am not 
aware that it ever faces northwardly of east and 
west, save through intrusion of other rocks. Its 
solidity varies sometimes within very confined limits, 
and this often depends on the sudden presence of 
quartz, which is seen to pervade the slate with great 
freedom in some spots. At times, there is reason to 
believe that access of air determines the decompo- 
sition of slate, though, in some cases where this is 
supposed to have happened, I should consider from 
its brown and powdery appearance that some pe- 
culiarity of chemical composition had existed 
previously. Generally in fact, the loose, rubbly 
condition in which we observe a deal of our slate 
to exist, cannot have been owing, at least in the 
Bs Wie a 
