PLUTONIC ROCKS OF SILURIAN PERIOD. 
573 
trades itself in veins, and nowhere covers the fossiliferous 
strata in large overlying masses, as is so commonly the pase 
with trappean formations.* 
'Now this granite, which is more modern than the Silurian 
strata of Norway, also sends veins in the same country into 
an ancient formation of gneiss; and the relations of the plu- 
tonic rock and the gneiss, at their junction, are full of inter¬ 
est when we duly consider the wide diiference of epoch which 
must have separated their origin. 
The length of this interval of time is attested by the fol¬ 
lowing facts: The fossiliferous, or Silurian, beds rest uncon- 
formably upon the truncated edges of the gneiss, the inclined 
strata of which had been denuded before the sedimentary 
beds were superimposed (see Fig. 621). The signs of denu- 
Fig. 621. 
Granite sending veins into Silurian strata and Gneiss. Christiania, Norway. 
a. Inclined gneiss, b. Silurian strata. 
dation are twofold; first, the surface of the gneiss is seen oc¬ 
casionally, on the removal of the newer beds containing or¬ 
ganic remains, to be w^orn and smoothed; secondly, pebbles 
of gneiss have been found in some of these Silurian strata. 
Between the origin, therefore, of the gneiss and the granite 
there intervened, first, the period when the strata of gneiss 
were denuded ; secondly, the period of the deposition of the 
Silurian deposits upon the denuded and inclined gneiss, a. 
Yet the granite produced after this long interval is often so 
intimately blended with the ancient gneiss, at the point of 
junction, that it is impossible to draw any other than an ar¬ 
bitrary line of separation between them; and where this is 
not the case, tortuous veins of granite pass freely through 
gneiss, ending sometimes in threads, as if the older rock had"^ 
offered no resistance to their passage. These appearances 
may probably be due to hydrothermal action (see below, p. 
584). I shall merely observe in this place that had such 
junctions alone been visible, and had we not learnt, from 
other sections, how long a period elapsed between the con¬ 
solidation of the gneiss and the injection of this granite, we 
might have suspected that the gneiss was scarcely solidified, 
* See the Gaea Norvegica and other works of Keilhau, with whom I ex¬ 
amined this country. 
