IGNEOUS ROCKS. 21 
one other circumstance, that of shelter, powerfully 
act on the climate, and restrain the other influences, 
which would tend to depression of the average heat. 
Again the foliage and wood of a forest, naturally 
keep of much ‘of the sun’s rays, and so allow the 
atmosphere within and about it, to remain unin- 
fluenced by its power; but, in opposition to that, 
the presence of a soil connected with a wooded 
state of a country, serves as a reservoir of solar heat, 
to be furnished to the air, in seasons when the sun’s 
power is lessened or withdrawn. Climate being 
deteriorated by moisture, that is to say, by the 
power of moisture in diminishing atmospheric heat, 
it cannot be surprising that the presence of much 
vegetation, is a powerful auxiliary in the ameliora- 
tion of the thermometric heat of a district, since 
this kind of carpet has the effect of appropriating 
a large proportion of water for continued sustenance. 
Impinging almost generally on the Dartmoor 
granite, and often closely joined to it,—and in one 
case (Black Aller Tor,) lying on it as a thin bed, 
or covering,—is Schzst of different kinds, a propor- 
tion of which puts on the mitcaceous character, 
and retains it for a short distance from the former 
rock, though mica slate is to be met with in one 
other spot, namely at the Bolt-head. ‘This mzca 
schist‘has favorable qualities for agriculture, es- 
pecially grain crops. 
A Serpentinous Rock has been noticed by Mr. 
Prideaux, extending through slate in the immediate 
vicinity of Dartmoor. 
_Shorl Rock in varying characters occurs also 
abundantly in contact with granite. Syenite near 
the Race House above Ivy Bridge. Trapp Rock 
in its several varieties pervades the county in the 
form of dykes, chiefly in connexion with lime, slate 
and sandstone, and conforming much to the direct- 
ion of their respective runs, where occurring. It is 
