

188 STRUCTURAL AND FIELD GEOLOGY 
laid upon it, yet this is not always the case. Occasionally, 
the eruptive rock seems to merge insensibly into the other, 
and no line of demarcation is visible. Again, it sometimes 
happens that when granite has invaded schists or slates it has 
penetrated these by a kind of leaf-by-leaf injection — the 
liquid rock having insinuated itself in excessively thin sheets 
and veins along planes of foliation or cleavage. Under such 
conditions the invaded rocks are so intimately mixed with 
granite and so highly metamorphosed, that it is often very 
difficult to distinguish between them and the invading rock, 
The alternating leaves of granite and schist combine, in 
short, to produce a rock which has the aspect of a gneiss 
into which the granite-mass seems, as it were, to graduate. 
Now and again the marginal area of a granitic batholith 
contains more or less numerous angular and subangular 
fragments, slabs, reefs, and blocks of schistose or other rocks, 
Such inclusions, or xenoliths, as they are called, are not to be 
confounded with the relatively fine-grained, dark basic secre- 
tions described in Chapter III., as characteristic of many 
granites. On the contrary, they have obviously been torn 
from the rocks abutting upon the granite and enclosed in it 
at the time of its intrusion. It may be added that granitic 
batholiths not infrequently show a kind of foliated or flow 
structure near their margins—the constituent minerals being 
arranged roughly parallel to the junction-line. This may 
indicate an actual fluidal movement, or it may simply be the 
result of hydrostatic pressure, exerted by the mass of the 
granite itself. 
Batholiths are often rudely circular or elliptical in groundplan, and 
seem, in some cases, to rise up vertically, as if they occupied an enormous 
pipe or funnel. The rocks surrounding such batholiths have no appear- 
ance of having been thrust aside to make room for the intrusive mass. 
To explain this, it has been suggested that the rocks which formerly 
occupied the site of the batholith may have been melted up and 
assimilated by the granite. That absorption to some extent has actually 
occurred, in some cases at least, is suggested by the fact, already 
mentioned, that granite occasionally merges gradually into the rocks 
against which it abuts. It is further noteworthy, in this connection, that 
a difference of chemical composition has now and again been detected 
between the granite near its margin and towards the centre of the mass. 
It would seem, however, that xenoliths are confined as a rule to the marginal 
areas of a batholith, whereas, had the rocks which formerly occupied its 
i eRe NZ. 

