re - me | “TER Fa 
. ~iil ‘ c ae. ¥ ne pe y 
= : 5 ' 3 ~ —e a in Yor 
s = X* 3 t - s epee 
bs ‘ a 4 ay 
‘ss 
550 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
marginal parts from many of the intrusive sheets in central 
Scotland, shows that even where no distinct glass remains the rock 
is crowded with black opaque microliths arranged in a delicate 
geometric network. Back from the surface of contact these 
microliths disappear, and the magnetite or titaniferous iron assumes 
its ordinary crystalline and often indeterminate or imperfect 
contours. Whether these bodies were developed only along the 
marginal portions of the intrusive mass and belong to conditions of - 
rapid cooling and escape of vapour, or were originally present as — 
incipient forms of crystallization throughout the entire rock, but 
have been lost in the subsequent growth of the crystalline forms, is - 
not quite clear, though the former supposition seems most probable. 
Another lithological characterstic of the intrusive as compared 
with the interbedded sheets is the considerable variety of composi-: 
tion and structure which may be detected in different portions of 
the same mass. A rock which at one place gives under the micro- 
scope a crystalline-granular texture, with the mineral elements of 
dolerite, will at a short distance show a coarsely crystalline texture 
with abundant orthoclase and free quartz—minerals which do not 
belong to normal dolerite. These differences, like those above 
referred to as noticeable among amorphous bosses, seem too local and 
sporadic to be satisfactorily referred to original differences in the 
composition of various parts of the molten magma, or to segregation 
by gravitation or otherwise. They suggest rather that great 
intrusive sheets have here and there involved and melted down 
portions of rocks, and have thus acquired locally an abnormal 
composition.’ : 
Hffects on Contiguous Rocks.—Admirable examples of 
the alteration produced by eruptive masses are not uncommonly pre- 
sented at the contact of intrusive sheets with the surrounding rocks. 
Induration, decoloration, fusion, the production of a prismatic struc- 
ture, conversion of coal into anthracite, of limestone into marble, 
and other alterations, may be observed. The nature of these changes 
is described at p. 572. 
Connection with Volcanic Action.—Many volcanic rocks 
occur in the form of intrusive sheets, as felsite, quartz-porphyry, 
diorite, melaphyre, diabase, dolerite, basalt, trachyte, and others. ‘The 
remarks above made regarding the connection of intrusive bosses 
with volcanic action may be repeated with even greater definiteness 
here. Intrusive sheets abound in old volcanic districts intimately 
associated with dykes and surface outflows, and thus bringing before 
our eyes traces of the underground mechanism of volcanoes. Inter- 
esting examples of this connection occur among the Carboniferous 
voleanic rocks of the basin of the Forth.? Many of the “ necks” or 
former volcanic vents are associated with intrusive sheets, which 
probably mark some of the subterranean protrusions of molten rock 
' Trans. Roy. Soc. Ldin. xxix. p. 492. Clough, Geol. Mag. 1880, p. 488. 
® See Trans. Roy. Soc, Edin, xxix, p. 474, 

