444 ; PROFESSOR GEIKIE ON THE 
the rock, by virtue of its joints, splits up into large angular blocks, it is exceed- 
ingly durable. Hence it has been well adapted for the formation of boulders, 
and, no doubt, from these hills many of the blocks of granitic conglomerate 
travelled, which are found scattered over the lowlands to the north and east 
of the Great Glen. Similar conglomerate reappears on the north side of the 
Beauly Firth to the north of Kessock Ferry. 
The geological structure and order of the strata in the tract of country 
between the Dornoch Firth and Inverness was well sketched in the early 
memoir by SepGwick and Murcuison. They showed that the margin of the 
Old Red Sandstone area is occupied by a very massive, coarse conglomerate, 
derived from the waste of the underlying and surrounding metamorphic rocks. 
This rock rises into conspicuous rounded hills, which run northwards almost to 
the borders of Caithness. The angle of dip appears to be generally very low, 
as we have seen to be the case in Caithness on the one hand, and along the 
south side of the Moray Firth on the other. Overlying the conglomerate, and 
passing down by intercalations into it, are dull red sandstones and flagstones, 
with reddish-grey bituminous and calcareous flagstones and shales. There can 
be little hesitation in identifying these strata with these which occupy a similar 
position at Clava and Culloden. They are seen on the Alness river “mixed 
with red and greenish-red marls;” on the Ault Graat, above the well-known 
gorge ; in the hills about Tulloch Castle, near Coul, and for a considerable way 
down Strathpeffer.* They appear to run northwards also for a long way, flank- 
ing the great marginal band of conglomerate. They have been detected by the 
Rev. Dr Joass of Golspie, at Edderton on the Dornoch Firth, and he has seen 
a tuberculated scale from the Brora district, which he is disposed to refer to 
Coccosteus. With regard to the Edderton locality, he has been kind enough 
to furnish me with the following particulars :—“The fossils there occur in 
calcareous nodules imbedded in two bands of red clay, interstratified with 
dark red sandstone, on the right bank of the Craig-roy Burn, about a mile above 
the bridge on the coast-road, which is half a mile from the Edderton Station. 
In the order of their abundance they were—Coccosteus, Osteolepis, Diplopterus, 
Cheiracanthus, Cheirolepis, Gilyptolepis (? Holoptychius Sedgwickit), Pterichthys, 
and Fucotds (2).” 
In these flaggy and ichthyolitic strata SEp@wick and Murcuison recognised 
the equivalents of the massive Caithness flagstone series. They are succeeded 
by an upper conglomerate band perhaps exceeding 300 feet in thickness. It 
is this rock through which the remarkable fissure-like ravine of the Ault Graat 
has been excavated. It passes up into red or reddish-grey sandstones, with 
occasional partings of grey shale. These overlying sandstones are seen in 
* Op. cit. p. 146. 

