2 
4. 


fear 62.) ARCHAAN. . 641 
with a much larger development of schists, and a great diminution in 
the quantity of pegmatite—characters particularly well seen at Gairloch. 
No satisfactory estimate has yet been made of the probable thickness of 
these rocks. On the lowest calculation it must amount to at least 
20,000 feet. 
Several features in the structure of this gneiss deserve attention. 
The pegmatite has been described as an intrusive granite traversing 
the gneiss in veins. In many cases, it is true, this rock looks as if it 
had been forcibly injected, for the foliation of the gneiss is abruptly 
_ bent up on either side of the pegmatite. But besides the difficulty of 
- conceiving that the coarsely crystalline materials of these veins ever 
could have been in such a state of igneous fusion or aquo-igneous 
plasticity as to be capable of being injected into rents of the surrounding 
rock, there are some characteristics which seem to make it nearly certain 
_ that the pegmatite belongs to the same series of crystalline processes by 
which the gneiss itself was produced. The same mass of pegmatite 
may be observed in one place regularly interbedded with the gneiss (Fig. 
315), and at another place traversing it in different directions (Fig. 316). 
But in both conditions there is the most intimate crystalline union of 
the pegmatite with the gneiss, the crystals of each rock dovetailing into 
each other. Here and there, too, the crumpled folia of gneiss pass into 
pegmatite, in which a rude crumpled foliation may be detected. At 
Cape Wrath, alternate thin layers of gneiss and pegmatite occur with 
as perfect regularity and as insensible gradations of structure as among 
the ordinary folia in any part of the gneiss (Fig. 315). But one of the 
































Fie. 315.—GNEISS WITH INTERSTRATIFIED BANDS OF PEGMATITE, CAPE WRATH. 
most singular facts remains to be noticed. In a number of examples 
from Cape Wrath to Loch Laxford I have observed that in pegmatite 
veins which cut across the gneiss, a rude foliation has been developed, 
parallel in a general sense to that of the gneiss on either side (Fig. 317). 
Such cases suggest that the pegmatite veins were produced before the 
process of foliation in the surrounding rock was completed, so that the 
“materials of the veins were to some extent affected by its later stages. 
_. Another conspicuous feature, especially of the lower massive gneiss, 
is the occurrence of geodes and lenticular bands or layers of black horn- 
blende or of a mixture of hornblende with a little felspar or quartz, less 
| AD 
