152 TRANSACTIONS OF THE PLYMOUTH INSTITUTION. 
Moreover, granite veins may be seen at many places thrust into 
the adjacent rocks, in a manner utterly irreconcilable with any 
metamorphic hypothesis. Again, the rocks bordering the granite 
all more or less exhibit traces of alteration, gradually dying away 
from the granitic margin, just as the granite should if it had been 
the result of a metamorphic process, and were not of independent 
origin. I put the inevitable conclusion in the words of Sir 
Warington Smith, " From these various indications ... it is clear 
that the slaty rocks . . . have been rudely disturbed and broken 
through by the granite, and that they are probably based upon a 
vast mass of that material lying at unknown depths between the 
hills where it rises into view." 
This identification of the Dartmoor granite as a true igneous 
rock is the first stage in our inquiry. Igneous rocks are commonly 
classed in two great divisions — volcanic, " such as have been 
ejected, or have welled out on the surface," and plutonic, "such as 
have been formed under conditions of depth and pressure." 3 
Petrological research has, however, made it absolutely certain that 
(and I use the words of Professor Prestwich rather than my own) 
this "in many cases indicates a difference of conditions rather than 
of origin. The volcanic rocks having erupted on the surface, and 
having cooled quickly and without pressure, have solidified as non- 
crystalline and amorphous rocks, whereas the so-called plutonic 
rocks, not having been erupted at the surface, and cooling slowly 
and under great pressure, have solidified as crystalline rocks." 4 
There is another classification of igneous rocks, according to 
their composition, into acidic and basic. These form two great 
series, in both of which plutonic and volcanic varieties are found. 
The acidic rocks have an average percentage of silica of 70 ; the 
basic of 50 ; and while alkalies are largely represented in the one, 
alkaline earths are conspicuous in the other. 
As long ago as 1874 Professor Judd, writing on the "Tertiary 
Volcanoes of the Highlands," 5 proved that in the acidic series 
the plutonic form, granite, passed upwards through syenitic 
granite into felsite (quartz -porphyry), then into trachytic-lava 
(felstone, or rhyolite), and finally into a volcanic glass (pitchstone). 
3 Prestwich's Geology, i. 35. 4 Geology, i. 381. 
5 Quar. Jour. Geol. Soc. xxx. 220, et seq. 
