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 :— 
P Acidic ... Granite Syeni 
LUTONIC } Basic ...  Diorite TELLS 
Acidic ...  Trachyte ) Porphyrite. 
VOLCANIC | Basic .. Basalt } Trachy-dolerite. 
Plutonic ... Granite Quartz porphyry. 
Acinic... | Volcanic ... Trachyte } Trachyte EEEa 
Plutonic ... Diorite Melaphyre, 
Basio --- | Volcanic .. Basalt } Diabase porphyrite. 
The former state of fusion of the Volcanic division of the izneo 
rocks is evident, as they may still be seen in process of formation £ us 
the lava of active volcanocs. Bom 
In the case of the other, or Plutonic division, their previ 
igneous fluidity is not always so apparent ; and it seems not improb ane 
that some granites, as well as other rocks of this class, are the =; it $ 
the gradual metamorphism, in place, of stratified sedimentar 4 oF 
through the effects of long-continued thermo-chemical action ue ae 
sence of water, and under high pressure, but unaccompanied i re- 
such great heat as would be required to cause igneous fluidit w any 
case it seems quite certain that many changes have taken nine in an 
character and composition of igneous rocks subsequently to ewe the 
formation, causing the introduction or development in them of re first 
termed accessory minerals, chiefly, no doubt, under the influ lat are 
watery solutions and gases penetrating and permeating them Ta or 
sense, nearly all rocks are, more or less, metamorphic ay „dn this 
words, have assumed different mineral characters from what he other 
sessed, either as original sediments or molten masses. TI ; 1ey pos- 
generally caused by the addition of new substances, but, in ma 
only through the Te-arrangement, under peculiar chemical pay sh cases, 
influences, according to their affinities, of certain original const; Physical 
the rock, resulting in the development of distinct “miner. ic Stituents of 
forming rocks, which, though differing greatly in char AT S, and thus 
would not vary more in their ultimate chemical consti tue to pearance, 
two distinct pieces of sandstone, slate, or granite. nts than any 
; GRANITE is the principal rock of the Plutonic divisio 
igneous rocks. It consists of a crystalline granular com n of the 
able proportions, of felspar, quartz, and mica. In certai eat 
occur chlorite, talc, hornblende, and schorl. The fels n vari 
clase, is usually the predominant ingredient, the mee 
smallest place in typical granite. It is uncertain 
i acidic 
s In vari- 
eties th 
f = ere 
w chiefly ortho 
& occupyin 
8 the 
whether albite or 
