- Parr VIII. § 2.) LOCAL METAMORPHISM. O73 
dyke, it ought to occur as frequently along joints in which there 
has been no ascent of igneous matter. 
Colouration.—Rocks, particularly shale and sandstone, in 
contact with intrusive sheets, are sometimes so reddened as to 
resemble the burnt shale from an ironwork. very case of 
reddening along a line of junction between an eruptive and non- 
eruptive rock, must not, however, be set down without examination 
as an effect of the mere heat of the injected mass, for sometimes the 
colouring may be due to subsequent oxidation of iron in one or both 
of the rocks by water percolating along the lines of contact. 
Induration.—One of the most common changes superinduced 
upon sedimentary rocks along their contact with intrusive masses is 
a hardening of their substance. Sandstone, for example, is converted 
into a compact substance which breaks with the lustrous fracture of 
quartzite. Argillaceous strata are altered into flinty slate, Lydian 
stone, jasper, or porcellanite. This change may sometimes be 
produced by mere dry heat, as when clay is baked. But probably 
in the majority of cases, induration of subterranean rocks results 
from the action of heated water. The most obvious examples of this 
action are those wherein the percentage of silica has been increased 
by the deposit of a siliceous cement in the interstices of the stone, 
or by the replacement of some of the mineral substances by silica. 
This is specially observable round eruptive masses of granite and 
some diabases.’ 
Expulsion of water.—One effect of the intrusion of molten 
matter among the ordinary cool rocks of the earth’s crust has 
doubtless often been temporarily to expel their interstitial water. 
The heat may even have been occasionally sufficient to drive off 
water of crystallization or of chemical combination. Mr. Sorby 
mentions that it has been able to dispel the water present in 
the minute fluid cavities of quartz in a sandstone invaded by 
dolerite.” 3 
Prismatie structure.—Contact with eruptive rocks has fre- 
quently produced a prismatic structure in the contiguous masses. 
Conspicuous illustrations of this change are displayed in sandstones 
through which dykes have risen (Fig. 297). Independently of the 
lines of stratification polygonal prisms, six inches or more in 
diameter, and several feet in length, starting from the face of the 
dyke, have been developed in the sandstone.* 
Some of the most perfect examples of superinduced prisms may 
occasionally be noticed in seams of coal which have been invaded by 
1 Kayser, on contact metamorphism amid the diabase of the Harz, Z. Deutsch. Geol. 
Ges. xxii. 103, where analyses showing the high percentage of silica are given. 
Hawes, Amer. Journ. Sci. January 1881. The phenomena of metamorphism round 
granite are further described below p. 578 seq. 
2 Q. J. Geol. Soc. 1880. 
% Sandstone altered by basalt, melaphyre, or allied rock, Wildenstein, near Biidingen, 
Upper Hesse; Schoberle, near Kriebitz, Bohemia; Johnsdorf, near Zittau, Saxony ; 
Bishopbriggs, near Glasgow. 
