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oat 
544 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. _ 
Connection of Granite with Volcanic. Rocks.—The 
manner in which some bosses of granite penetrate the rocks among 
which they occur strongly recalls the structure of volcanic necks or 
pipes. The granite is found as a circular or elliptical mass which 
seems to descend vertically through the surrounding rocks without 
seriously altering or disturbing them, as if a tube-shaped opening 
had been blown out of the crust of the earth up which the granite 
had risen. Several of the granite masses of the south of Scotland 
above referred to exhibit this character very strikingly (Fig. 271). 
That granite and granitoid rocks have probably been associated with 
volcanic action is indicated by the way in which they occur in con- 
nection with the Tertiary volcanic rocks of Skye, Mull, and other 
islands in the Inner Hebrides. Mr. Jukes suggested many years 
ago that granite or granitoid masses may he at the roots of volcanoes 
and may be the source whence the more silicated lavas proceed.’ 
Metamorphic Origin of some Granite.—tThe association 
of volcanic action with metamorphism has been already referred to 
(p. 508). While the instances are few where any satisfactory con- — 
nection can actually be traced between granitic masses and true 
lava-form or volcanic rocks, the close relationship between granite 
and the crystalline schists has long been recognized. Leaving forthe 
present the problem of the origin of these schists, it must be admitted 
that in some instances at least gneissoid and schistose rocks are the 
results of the metamorphism of mechanically formed sedimentary 
strata. That the granite associated with such rocks is of meta- 
morphic origin, that is to say, has been produced by the gradual 
softening and recrystallization of other rocks at some depth within 
the crust of the earth, seems in the highest degree probable. ‘This 
granite is associated with gneiss in such a way as to suggest that both 
have had a common origin, and as gradations can be traced from 
this gneiss through less distinctly crystalline schists into unaltered 
strata, such granite may be looked upon as the extreme of meta- 
morphism, the various schists and gneisses being less advanced 
stages of the process (p. 578, seg.). Provided the chemical composition 
of the altered rock be similar to that of granite, it is not necessary 
that the granite resulting from its alteration should be supposed to 
differ in any noteworthy particular from eruptive granite. The 
members of the Geological Survey of Ireland have indeed distin- 
guished two granites in Galway, one of which (characterized by the 
occurrence of orthoclase and oligoclase) they regard as metamorphic, 
the other (with orthoclase only) as igneous. More recently in the — 
east of the island they have separated two groups of granites, of 
which the intrusive masses are composed of dark-coloured quartz, 
orthoclase, albite, and black mica (Mourne Mountains), while the 
metamorphic variety is formed of grey felspar, quartz, and black 
mica. 
' Manual of Geology, 2nd ed. p. 93; Geikie, Trans, Geol. Soc. Edin. ii. p. 8301; Judd 
Quart. Journ, Geol, Soc, xxx. p. 220; Reyer, “ Beitrag zur Physik der Eruptionen,” 

