THE PRE- CAMBRIAN ROCKS, BRITISH ISLES. 7 



sive crystalline protrusions have been converted into perfect 

 schists. The dykes of dolerite have been transformed into horn- 

 blende-schists and the granitic pegmatites have been reduced to 

 a kind of powder which has been rolled out so as to simulate the 

 flow-structure of a lava. There is evidence that, most, if not all, 

 of this dynamical change was effected long before the deposition 

 of the Torridonian series, for the latter rests in nearly horizontal 

 sheets, with a strong unconformability upon the crushed and 

 sheared gneiss. 



Torridoii Sandstone. This group of rocks covers only a limited 

 area in the north-west of Scotland, but it must once have spread 

 over a far more extensive region. It reaches a thickness, as I 

 have said, of 8,000 or 10,000 feet, and consists almost wholly of 

 dull, purplish-red sandstones, often pebbly, and bands of con- 

 glomerate. Dark grey shales, already alluded to as occurring 

 towards the base of the series, are repeated also in the highest 

 visible portion, and have yielded tracks of what seem to have 

 been annelids and casts of nail-like bodies which may have been 

 organic. I have said that the Torridonian deposits which were 

 classed by Murchison as Cambrian, have been proved by the dis- 

 covery of the Olenellus zone in an unconformable position above 

 them, to be of pre-Cambrian age. Except along the line of dis- 

 turbance to which I shall immediately refer, these strata are quite 

 unaltered. Indeed, in general aspect they look as young as the 

 old red sandstones with which Hugh Miller identified them. It 

 is at first hard to believe that such flat undisturbed sandstones 

 are of higher antiquity than the very oldest Palaeozoic strata 

 which are so generally plicated and cleaved. 



The interval of time between the deposition of the Torridon 

 Sandstone and of the overlying Cambrian formations must have 

 been of enormous duration, for the unconformability is so vio- 

 lent that the lowest Cambrian strata, not only transgressively 

 overspread all the Torridonian horizons, but even lie here and 

 there directly on the old gneiss, the whole of the intervening thick 

 mass of sandstone having been there removed by previous denuda- 

 tion. At Durness, in the north of Sutherland, about 2000 feet of 



