PRESIDENTIAL ADDRESS. 451] 
organisms are worm casts of the nature of Planolites, although the limestone 
and chert may have originated from the débris of the calcareous and siliceous 
organisms of the plankton. A noticeable feature of the Ghrudhaidh and Hilean 
Dubh groups is the occurrence in them of bands of brecciated dolomite on several 
horizons, which do not imply any break in the continuous sequence of deposits. 
The total thickness of this portion of the Durness dolomites and limestones, 
yielding no fossils beyond worm casts, amounts to 350 feet. 
But in the upper part of the Sail Mhor group siliceous and calcareous 
organisms of a higher grade make their appearance. Among the former we find 
the Zthabdaria of Billings. The calcareous forms are represented by (1) gastero- 
pods, including a single specimen of a murchisonid, and two species ’of a pleuro- 
tomarid (Huconia Ramsayi and H. Htna) of a type occurring in the calciferous 
rocks of Newfoundland and Canada; (2) cephalopods, comprising two slightly 
bent forms with closely set septa and wide endogastric siphuncles, showing 
affinities with those of Hndoceras and Piloceras; (3) arthropods, represented by 
the epitome of a large asaphoid trilobite resembling that of Asaphus canalis of 
Conrad. This evidence is insufficient to determine the exact horizon of these 
beds, but clearly indicates that we are no longer dealing with Lower Cambrian 
strata. The cephalopods are like those found in the Ozarkic division of Ulrich 
(Upper Cambrian), in North America. According to Schuchert, the cephalopods 
with closely set septa are of Cambrian type and older than those of the Beek- 
mantown terrane of American geologists. On the other hand, the asaphoid type 
of trilobite is suggestive of a somewhat higher horizon. 
No fossils have been found in the overlying Sangomore group, about 200 feet 
thick, which consists mainly of granular dolomite, with bands of chert, some 
‘being oélitic, together with thin fine-grained limestones near the top. 
Above this horizon, at a height of over 800 feet above the top of the Olenellus 
zone, we encounter the great home of the fossils peculiar to the Durness lime- 
stone in the Balnakeil and Croisaphuill groups. The former consists mostly of 
dark limestones, with nodules of chert, and, with a few alternations, of white 
limestone bands. A few thin layers are charged with worm casts. The over- 
lying group is more varied, the lower part being composed of dark grey lime- 
stones full of worm casts, and with some small chert nodules arranged in lines; 
the middle portion, of dark granular and unfossiliferous dolomite; and the upper 
part, of massive sheets of fossiliferous limestone full of worm casts. The total 
thickness of these two groups in Durness is about 550 feet. 
These two subdivisions have yielded over twenty genera and about one 
hundred species. In Durness sixty-six species have been obtained from the 
Balnakeil group alone, fifteen of which have not as yet been found in the over- 
lying Croisaphuill group, thus leaving fifty-one species common to both 
divisions. The Ben Suardal limestones in Skye, which were mapped by the 
Geological Survey as one division, are regarded, on paleontological grounds, as 
the equivalents of both these groups. Owing to the number of species common 
to both subdivisions, the fauna will be here referred to as a whole. 
Both siliceous and calcareous organisms are present in this fauna. Among 
the former we find Archeoscyphia (Hinde), described by Billings as Archeo- 
cyathus, an early Cambrian coral, but shown by Hinde to be a siliceous sponge.4 
The genus Calathium is represented by four species. Other genera and species 
of sponges occur, so that the siliceous nodules, which are very common in both 
groups, may be in great part due to them. In this connection it may be men- 
tioned that Hinde obtained sponge spicules from some of the nodules. Hinged 
brachiopods have also been collected from these beds and include Nisusia 
(Orthosina) festinata, N. grandeva, and Camarella. 
But the characteristic feature of the fauna is the assemblage of calcareous 
mollusca comprising lamellibranchs, gasteropods, and cephalopods, showing a 
wide range of variation, and consequently a long ancestry. The lamellibranchs, 
though represented only by two genera, Huchasma and Hopteria of Billings, with 
several intermediate forms, are of extreme interest, as they are only known to 
occur elsewhere in Newfoundland and Eastern Canada. The gasteropods, how- 
ever, furnish the largest number of species—about 48 per cent. of the whole. 
The primitive euomphalids, Maclurea and Ophileta, are most characteristic. 
* Quart. Jour. Geol. Soc., vol. xlv., p. 125, 1889. 
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