270 Pre-Cambrian Rocks of Ross-shire. 
pale in colour, often colourless, and is recognized in thin section by its 
crystal outline and the absence of double refraction. It is probably a 
lime-alumina garnet, and is more evenly distributed than the sphene. 
Amongst the rocks from Ben Fyn I found none which could be 
grouped with those described in Notes 15—20, of the upper series, the 
nearest approach to the most highly altered of which being a specimen 
from Gaerloch (Note 1), and some fragments from the conglomerate 
from that neighbourhood ; though in these cases the crystallization is 
still more distinct than in any of those of the newer series, whilst the 
presence of both sphene and garnet serves to show its true relations. 
The foliation of the gneisses, although rendered more distinct by 
the presence of the mica bands, is by no means wholly dependent 
upon the parallel arrangement of the lamine of this constituent, this 
character being often contributed to in a marked degree by the form 
of the quartz in the quartzo-felspathic bands (v. Notes 2 and7). On the 
contrary, the schistosity of the quartzites of the upper series (often 
miscalled gneiss) is entirely due to this parallelism of the mica flakes, 
the remainder of the rock, even where felspar is present, showing few 
or no indications of the foliation of the gneiss. Sphene very rarely 
occurs in the quartzites (however micaceous), both this and the 
garnet being always fragmentary. 
The bands of hornblende rocks in the gneiss from Ben Fyn are 
closely similar to those of Gaerloch and from other western areas 
which I have examined, and they contain abundance of sphene. It 
may be noted en passant that many of the rocks hitherto designated 
hornblende-gneisses contain no trace of hornblende, the black mica 
having probably been mistaken, in the majority of cases, for this 
mineral. ‘That it occasionally occurs there can be no doubt; but it 
does not appear to be in any way a characteristic of the gneisses of 
- this area. 
The intergrowth of biotite and muscovite observed in one of the 
specimens from Ben Fyn—recalling to mind the not-infrequent 
interlamination of two species of chlorite—was new to me; but I 
have since found that Zirkel has called attention to such an 
occurrence in an Archean gneiss from the Ogden Canon, 
Wahsatch Mountains, Utah, U.S.A.‘—T.D.] 
General Conclusions. 
The evidence given tends to show conclusively that a floor of 
Pre-Cambrian rocks can be traced almost continuously along certain 
lines from the west coast to the central mountains of Ross-shire. 
That it attains in some cases to heights of over 5000 feet above the 
present sea-level, whilst at others it descends to that level, and even 
below it. It has avery uneven surface, but the main depressions 
are along well-defined lines. The most marked being those which 
trend in lines from N.E. to 8.W. In these depressions newer sedi- 
ments have accumulated, chiefly rather shallow-water deposits, and 
such as would be derived by the denudation of the rocks composing 
the floor. Some, however, there are like the limestones, which must 
1 Microscopical Petrography, F. Zirkel, p. 24. 
