52 



NATURE 



[September 12, 1912 



Ophileta are found, some of which likewise occur in 

 the Beekmantown limestone. Euomphalus has also 

 been recorded, while several forms belonging to the 

 nearly allied family of the Turbinidae, and placed in 

 Lingstrom's genus Oriosioma, are also met with in the 

 Beekmantown limestone. 



Murchisonids and Pleurotomarids number twenty- 

 seven species and show a very wide range of variation. 

 The chief subgenera of the former are Hormotoma 

 and Ectomaria, many species of which occur with 

 remarkable variations. All the types of variation 

 found in Durness are to be found in North America, 

 and several of the species are common to both regions. 

 The pleurotonjarids vary in a similar manner, the chief 

 genera being Raphistoina and Euconia, and a form 

 resembling Hormotoma, only with a shorter spire. 

 Species belonging to each of these subgenera are Jike- 

 wise common to both areas, while some are only 

 known from the north-west Highlands. 



The cephalopods are of equal interest. They are 

 also of primitive type, and, at the same time, show a 

 wide range in form. The prominent feature in the 

 straighter specimens is the great width of the laterally 

 placed siphuncle, which is generally furnished with 

 endocones and organic deposits. The genus Piloceras 

 is the most characteristic type and shows this peculiar 

 feature best. It has only been recorded from Scot- 

 land, Newfoundland, Canada, and the eastern States 

 of North .America. The following additional genera 

 are represented, viz. Endoceras, chiefly by siphuncles 

 in great variety; Aclinoceras, Cyrtoceras, and, doubt- 

 fully, Ortlioceras. Several forms have been attributed 

 to Orthoceras which, on re-examination have been 

 found to be the siphuncles of other genera, resembling 

 American types described by Hall and Whitfield. 



The whorled nautiloids provisionally classed with 

 the genus Trocholites of Conrad are represented by 

 several distinct forms as yet unnamed. 



The trilobites are of rare occurrence in these two 

 groups of dolomite and limestone. They are frag- 

 mentary and poorly preserved. This is doubtless one 

 of the disappointing features connected with this re- 

 markable assemblage of organic remains, for the 

 jiresence of a zonal form would have helped to define 

 the horizon of these beds. Only one species, Batliynis 

 Scro (Billings) has been identified, which also occurs 

 in the Beekmantown limestone of Newfoundland. The 

 other trilobite remains, though poorly preserved, leave 

 a Cambrian facies characteristic of North .-Xmerica. 



In connection with this fauna certain features have 

 been observed which throw some light on the absence 

 of calcareous organisms from thick zones of the 

 Durness dolomite and limestone. In my detailed de- 

 scription of the palaeontology of the Cambrian rocks 

 of the north-west Highlands in the Geological Survey 

 Memoir I stated that "in most cases the septa and 

 walls of chambered shells have been wholly or in part 

 dissolved away, so as to leave only the more massive 

 structures of the siphuncles, and worm castings are 

 often found within the chambers where the septa have 

 been preserved. These features seem to indicate that 

 the accumulation of the calcareous mud in which the 

 fossils were embedded was so slow that there was 

 time for the solution of part of an organism before 

 the whole of it was covered up." ' There is good 

 reason to believe that many organisms w-holly dis- 

 appeared by this process, so that it is reasonable to 

 conclude that the fossils obtained from the Durness 

 dolomites cannot be regarded as furnishing a complete 

 life-history of the forms that originally existed in that 

 sequence of deposits. Attention has already been 

 directed to the fact that beneath the two subdivisions 

 now under consideration there are groups of dolomite 



•' "Geological Structure of ihe Norih-west Highlands," Geol. Sur. Mem., 

 IQ07, p. 380. 



NO. 2237, VOL. 90] 



and limestone which so far have yielded no organic 

 remains beyond worm castings. And even in the 

 important Croisaphuill group, with its fossiliferous 

 zones, there are thick groups of dolomite which have 

 furnished no calcareous organic remains. Obviously 

 the palaeontological record in this instance is glaringly 

 incomplete, for we have no reason to suppose that the 

 life of the time flourished in some of the calcareous 

 zones and not in others. 



The highest subdivision of the Durness limestone, 

 measuring about 150 feet in thickness (Durine group), 

 has yielded two species of Hormotoma — viz. H. 

 gracilis and H. gracillima — both of which occur in the 

 two underlying groups. H. gracilis occurs in the 

 Beekmantown, the Chazy, and the Trenton limestones 

 of -America. 



Before assigning any stratigraphical horizons to the 

 fauna of the Durness dolomites, it is desirable, owing 

 to the American facies of the fossils, to recapitulate 

 the evidence bearing upon the life of Cambrian time 

 in North America. But the Cambrian life-history of 

 Scotland would be incomplete without a brief refer- 

 ence to the recent discovery of fossils along the eastern 

 border of the Highlands. 



In igii Dr. Campbell announced in The Geological 

 Magazine that fossils had been found in the Highland 

 border series north of Stonehaven, and, during this 

 year. Dr. Jehu made a similar discovery in rocks 

 belonging to this series near Aberfoyle. Papers on 

 these subjects will be communicated to this section. 

 For my present purpose it will be suflicient to indicate 

 the nature of the fossils and the lithological characters 

 of the rocks containing them. 



The Highland border series north tif Stonehaven 

 and near Aberfoyle includes sheared igneous rocks, 

 both lavaform and intrusive, with black shales, cherts, 

 and jaspers. North of Stonehaven the fossils occur in 

 thin, dark, flinty pyritous shale, vihile at Aberfoyle 

 they have been found in shaly films at the edge of the 

 chert bands. Several years ago radiolaria were de- 

 tected in the cherts between Aberfoyle and Loch 

 Lomond. From time to time these Highland border 

 rocks have been carefully searched for fossils, but 

 until recently with little success, owing to the intense 

 movement to which they have been subjected, result- 

 ing in marked flaser structure in all except the most 

 resistant bands. 



The fossils consist chiefly of horny, hingeless 

 brachiopods, phyllocarid Crustacea, worm-tubes, and 

 the jaws and chetae of annelids. The genera of 

 brachiopods comprise Lingulclla, Oboliis, Obolella, 

 Acrotreta, and Linarssonia. The association of these 

 brachiopods with phyllocarid crustaceans resembling 

 Hymenocaris and Lingulocaris is suggestive of an 

 Upper Cambrian horizon — an inference which is sup- 

 ported by the absence of graptolites. 



In the published Geological Survey maps these 

 Highland border rocks are queried as of Lower 

 .Silurian age. This correlation was based partly on 

 their resemblance to the .Xrenig volcanic rocks and 

 radiolarian cherts of the Southern Uplands, and partly 

 because, as shown by Mr. Barrow, they are overlain 

 by an unconformable group of sediments, termed by 

 him the Margie series. The cherts, the green schists, 

 and the Margie series have shared in a common 

 system of folding, and are unconformably surmounted 

 by Downtonian strata near Stonehaven. Though the 

 original correlation may not be strictly correct, it is 

 probable, in my opinion, that representatives of both 

 the Arenig and LTpper Cambrian formations may occur 

 in the Highland border series, and, further, that 

 Upper Cambrian strata may yet be found in the 

 Girvan area, as originally suggested by Professor 

 Lapworth in correspondence with Dr. Home. 



