450 TRANSACTIONS OF SECTION C. 
But the organic remains that render the fucoid beds of exceptional interest 
and importance are the trilobites, because they clearly define the horizon of 
this zone in the Cambrian system and display strong affinities with American 
types. They are aera by five species and varieties of Olenellus, very 
closely resembling the forms in the Georgian terrane, or Olenellus zone, on the 
east and west sides of the North American continent. The genus Olenelloides 
has also been recorded from these beds. The crustacea are represented by 
phyllocarids, among which we find Aristozoe rotundata, likewise characteristic of 
the Olenellus zone of North America. 
Next in order comes the serpulite grit, which indicates a recrudescence of 
the pipe-rock conditions of deposition, and presents the Scolithus type of 
annelid borings. From the diameter of the pipe and the depth of the burrow it 
is probable that the worm may have belonged to a different species from any 
of those whose casts are to be found in lower horizons. This large variety is 
associated with smaller and more irregular worm casts which have often 
weathered out and left the rock honeycombed with hollow casts. The 
characteristic form from which the zone takes its name is Salterella (Serpulites 
Maccullochit). It occurs abundantly along certain calcareous layers that mark 
pauses in the deposition of the sand. This calcareous type culminates at the top 
of the zone, where there is a thick, carious, weathering band, crowded with 
specimens of Saltere/la, forming a passage bed into the calcareous shales at the 
base of the Durness dolomites. At one locality near Loch an Nid, Dundonnell 
Forest, Ross-shire, thin shales, intercalated in the serpulite grit, yielded a fine 
carapace of Olenellus Lapworthi—a form of frequent occurrence in the under- 
lying fucoid beds. Professor Lapworth recorded the finding of Orthoceras and 
linguloid shells in the top part of this zone at Eireboll.® 
Immediately above the serpulite grit in Eireboll and Assynt we find a few 
feet of dark calcareous shale, with iron pyrites, probably deposited at the limit 
of sedimentation. This layer, which is singularly devoid of organisms, ushers 
in the great succession of dolomites and limestones, upwards of 1,500 feet in 
thickness—perhaps the most remarkable type of sedimentation among the 
Cambrian rocks of the North-West Highlands. The Geological Survey has 
divided this calcareous sequence into seven well-marked groups, some of which 
have as yet yielded no fossils beyond worm casts. Attention will presently be 
directed to the absence of calcareous forms in many of the bands of dolomite and 
to the probable cause of their disappearance. 
The thin calcareous shale just referred to is followed by dark blue dolomite 
limestone, forming the basal portion of the Ghrudhaidh group. It contains 
sparsely scattered, well-rounded sand grains, with a bed about three feet thick, 
near the bottom, charged with Salterella pulchella and S. rugosa. In the over- 
lying twenty feet of dolomite the sand grains gradually disappear, and the rock 
assumes a mottled character, due to innumerable worm casts of the Planolites 
type. Here a second layer, yielding Salterclla pulchella and 8. rugosa, super- 
venes, both forms occurring in the Olenellus zone of North America. 
The brief summary of the paleontological evidence which has just been given 
clearly shows that the strata ranging from the middle of the pipe-rock zone to the 
upper Salterella band of the Durness dolomites represent in whole or in part the 
Olenclius zone of North America. Owing to the absence of fossils we have no 
means of deciding more definitely the base and top of the Lower Cambrian rocks 
of the North-West Highlands. All the quartzites lying below the middle of the 
pipe-rock, notwithstanding the absence of zonal forms, have been included in 
the Lower Cambrian division. This correlation receives some support from the 
remarkable discovery of Dr. Walcott, who found primitive trilobites several 
thousand feet beneath the beds yielding Olenellus Gilberti, the form closely 
allied to the Highland trilobites. 
On the other hand, when we pass upwards for a certain distance from the 
Salterella bands the evidence is insufficient to establish the stratigraphical 
horizon of the beds. For in the overlying strata, comprising the remainder 
of the Ghrudhaidh group, the whole of the Eilean Dubh group, and the lower part 
of the Sail Mhor group, and consisting of dolomites, limestones, and cherts, with 
little or no terrigenous material, the only fossils that can be shown to be due to 
* Geol. Mag., vol. x., new series, p. 126, 1883. 
