404 
THE TERTIARY VOLCANOES 
BOOK VIII 
bedded basalts once covered them is indicated by the position of the three 
outliers of the basalt-plateau already noticed. But a fourth outlier still lies 
upon the porphyry of Orval as a cake that dips gently northward. It con- 
sists of a bedded, dark, finely-crystalline, opliitic dolerite, porphyritic in 
places, with a rudely prismatic or columnar structure (Fig. 360). It has 
Fig. 360. — Section on nortli side of Orval, Hum. 
a, Torridon sandstones ; b, bedded basalts of Fionn Cliro; e, dolerite ; d, quartz-porphyry. 
undergone contact-metamorphism, and tongues from the underlying rock 
project up into it. On the south-eastern side of the same hill, still more 
striking evidence is presented of the posteriority of the acid to the basic 
rocks. The porphyry shows here the same tendency to assume a bedded struc- 
ture, the parallel “ beds ” again dipping outward or south-east at 40°. They 
plunge under the body of gabbro, dolerite and other intrusive masses which 
from this point stretch eastward into the great cones of Allival and its 
neighbours. The rock at the junction is a fine microgranite with traces of 
micropegmatite. It is composed of a holo-crystalline base of quartz and 
orthoclase, with porphyritic crystals of microcline, blebs of quartz and 
scattered granules of augite. The rocks that rest immediately next it are 
basalt and dolerite, into which it has sent an intricate network of veins 
Fig. 361. — Junction of Quartz-Porphyry (Microgranite) and Basic Hocks, south-east side of Orval, Runl- 
et, basalts and dolerites ; b, dolerite and gabbro veins ; c, quartz-porphyry cutting a and b. 
(Fig. 361). 1 It has also pushed long tongues down the slope into them, 
which may be seen traversing the dolerite and gabbro veins that 
cut the basalts. The basic rocks next the porphyry have been intensely 
1 In a thin slice cut from a specimen showing the junction, there is a minute vein of the 
porphyry penetrating the basalt which is much altered, while the porphyry becomes much fin er 
in grain than at a distance from the contact. 
