468 Notices of Memoirs— Dundee, 1912— 



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 3 feet thick), near the bottom, charged with Salterella 

 pulchella and S. rugosa. In the overlying 20 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 Salterella pulchella and 8. rugosa, super- 

 venes, both forms occurring in the Olenellus zone of North America. 



The brief summary of the palseontological 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 Olenellus 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 strati- 

 graphical 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 organisms are worm 

 casts of the nature of Planolites, although the limestone and chert may 

 have originated from the debris of the calcareous and siliceous organisms 

 of the plankton. A noticeable feature of the Ghrudhaidh and Eilean 

 Dubh groups is the occurrence in them of bands of bre?ciated 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 fosiils beyond worm ca?ts, 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 Rhabdaria of Billings. The calcareous forms are repre- 

 sented by (1) Gasteropods, including a single specimen of a Murchisonid, 

 two species of a Pleurotomarid {Euconiu Ramsayi and E. Etna) 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 Endoceras 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 Beekmantown 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 oolitic, together with thin fine-grained limestones 

 near the top. 



