2 IGNEOUS ROCKS. 
The ‘separation between the two groups cannot be very rigidly 
carried out, as certain rocks of each group vary so greatly in their com- 
position as actually to graduate into the opposite one. This is also 
the case as regards the Plutonic and Volcanic divisions, there being no 
definite depth of measurement that can be fixed as a boundary between 
these two kinds of formations: thus, each of the four groups may be 
characterized by some typical rock, and each may also be connected 
with the other group by means of rocks of an intermediate character 
and origin. 
The groups may be represented as follows gos 
Acidic --. Granite ; 
PRIDE i Basic ... _ Diorite \ ania 
' “*% Acidic - -... Trachyte ) Porphyrite. 
Vanesa Basic ... ‘Basalt } Trachy-dolerite. 
Neate Plutonic .... Granite Quartz porphyry. 
"| Volcanic -... Trachyte f Trachyte porphyry. 
BASO f Plutonic T Diorite }' Melaphyre. 
“| Voleanic ... Basalt Diabase nadiye 
The former state of fusion of the Volcanic division of the igneous 
rocks is evident, as they may still be seen in process of formation from 
the lava of active volcanoes. 
In the case of the other, or Plutonic division, their previous 
igneous fluidity is not always so apparent; and it seems not improbable 
that some granites, as well as other rocks of this class, are the result of 
the gradual’ metamorphism, in place, of stratified sedimentary rocks, 
through. the effects of long-continued thermo-chemical action, in pre- 
sence of water, and under high pressure, but unaccompanied by any 
such great heat as would be required to cause igneous fluidity. In any 
case it seems quite certain that many changes have taken place in the 
character and composition of igneous rocks subsequently to their first 
formation, causing the introduction or development in them of what are 
termed accessory minerals, chiefly, no doubt, under the influence of 
watery solutions and gases penetrating and permeating them. In this 
sense, neatly all rocks are, more or less, metamorphic, or, in other 
words, have assumed different mineral’ characters from what they pos- 
sessed, either as original sediments or molten masses. This is not 
generally caused by the addition of new substances, but, in most cases, 
only through the re-arrangement, under peculiar chemical and physical 
influences, according to their affinities, of certain original constituents of 
the rock, resulting in the development -of distinct minerals, and thus 
forming rocks, which, though differing greatly in external appearance, 
would not vary more in their ultimate chemical constituents than any 
_ two distinct pieces of sandstone, slate, or granite. 
GRANITE is the principal rock of the Plutonic division of the acidic 
igneous rocks. It consists of a crystalline granular compound, in vari- 
able proportions, of felspar, quartz, and mica. In certain varieties there 
occur chlorite, tale, hornblende, and schorl. The felspar, chiefly ortho- 
clase, is usually the predominant ingredient, the mica occupying the 
_ smallest place in typical granite. It is uncertain whether albite or 
