

216 STRUCTURAL AND FIELD GEOLOGY 
rocks. Thus coals may become coked at a distance of many yards from 
a basalt, while the intervening sandstones and shales may show little or 
no change beyond slight induration or discoloration. Limestone is 
likewise somewhat readily influenced by basalt—the rock becoming 
converted into a crystalline marble, for a few feet or more from the line 
of contact. 
But the most notable contact metamorphism is induced by great 
plutonic batholiths—more especially by granite. The phenomena are 
perhaps most conspicuously displayed in places where the rocks surround- 
ing a granite consist of what were originally more or less unaltered 
greywacké and shale, or other strata of derivative origin. In such a 
region one can study all the various modifications which the strata 
undergo, as they are followed towards their contact with the eruptive 
mass. The zone or aureole of altered rocks surrounding a large batholith 
of granite may be a mile or more in width (see Fig. 79). Along the 

FIG. 79.—BATHOLITH WITH AUREOLE OF METAMORPHOSED ROCKS. 
g, granite; gw, greywackés and shales ; m, metamorphosed rocks. 
outer margin of this zone, clastic rocks begin to show more or less notable 
evidence of induration. In these indurated but otherwise unaltered rocks, 
the changes produced depend largely, as we have seen, on their 
mineralogical and chemical composition. Should the rocks be essentially 
argillaceous, aluminous silicates, such as chiastolite, often make their 
appearance, while at the same time biotite may be developed. Occa- 
sionally, when carbonaceous matter is diffused through a shale or slate 
this may become aggregated to form more or less abundant dark spots, 
and so give rise to one type of the rock known as spotted slate (see Plate 
XXII. 1). These carbonaceous spots disappear as metamorphic change 
increases and a schistose structure is superinduced. In other cases the 
spots take the form of concretionary knots, which seem to consist 
essentially of micaceous matter, cordierite, or andalusite. Knotted or 
spotted slates of this kind usually contain other new minerals, such as 
dt LCR ERIS 

