September 17, 1891] 



NATURE 



479 



Thraen in Astronomische Nachrichten, No. 3054, shows that 

 the comet crosses the equator near the end of October : — 



1891 



Sept. 



Oct. 



Brightness. 

 9-1 



Nov. 



It will be seen that the comet 

 at the date of discovery ( May 4) . 

 be reached about October 19. 



10-4 



1 now nine times brighter than 

 The maximum brightness will 



GEOLOGY AT THE BRITISH ASSOCIATION. 



'T'HE Address of the President of the Geological Section 

 -'■ having been devoted to the general questions involved in 

 the origin, association, and working of coal, it was natural that 

 other papers on the economic side of the science should claim 

 considerable interest. Prof. Boyd Dawkins stated that the 

 Channel Tunnel boring had been carried to a depth of 1500 feet, 

 with the result of penetrating coal-measures dipping gently to 

 the south at 1 1 13 feet. Six seams, containing 10 feet of work- 

 able coal, had been pierced between that depth and the present 

 bottom of the boring. The author endeavoured to show the 

 probability that a thick series of coal-measures, with workable 

 coals like those of Liege on one side and Somerset on the other, 

 would be met with if the boring were continued, and pointed 

 out the advantage possessed by the south-eastern coal-field in its 

 moderate depth and the comparatively uncrushed character of 

 the coal. 



In an exhaustive paper Mr. Topley summarized the chief 

 facts bearing on the origin of petroleum. He pointed out that, 

 while the American oil was mainly derived from Falasozoic 

 rocks, that in Europe and Asia came largely from Secondary 

 beds, and the large Caucasian supply was drawn from rocks 

 of Miocene age. 'I'he essential conditions for the supply of oil 

 appeared to be, a porous rock, generally of sandstone or lime- 

 stone, which served as a reservoir and was underlain by or 

 contained beds largely consisting of organic remains, with an 

 impervious cover of shale. In many cases the limestone had 

 been dolomitized and transformed into a cavernous rock which 

 was capable of storing the gas and oil. Such rocks can contain 

 from one-eighth to one-tenth of their bulk of oil. The oil was 

 driven to the surface by artesian pressure, and so gas was 

 generally met with on the summits of anticlines and oil on their 

 flanks. Where the rocks were very highly disturbed oil occurred, 

 but not in very great abundance, while gas was rarely found. 



Mr. Ross, in a paper on the same subject, endeavoured 

 to prove that the oil was mainly generated by the action of 

 solfataric volcanic energy upon beds of limestone, basing his con- 

 clusion on the occurrence of hydrocarbon and sulphurous vapours 

 in solfataras, and the constant association of rock salt, dolomite, 



and gypsum with the rocks yielding petroleum. He exhibited 

 equations to show that the action of sulphur dioxide and sulphu- 

 retted hydrogen on carbonate of lime, with or without water 

 and peroxide of hydrogen, was capable of producing the ethylene 

 and marsh gas derivatives, and he quoted experiments of Bischof 

 to show that sulphur was formed by similar reactions, arguing 

 that the hydrocarbons must be necessary by-products. 



Sir Archibald Geikie communicated two most important 

 papers on the results of Geological Survey work in the North- 

 western Higlands. One of these papers, relating to the discovery 

 of the Olenellus zone in the North-west Highlands, was as fol- 

 lows : — "Ever since the Geological Survey began the detailed 

 investigation of the structure of the North-west Highlands of 

 Scotland, the attention of its officers has been continuously 

 given to the detection of any fossil evidence that would more 

 clearly fix the geological horizons of the various sedimentary 

 formations which overlie the Lewisian gneiss. A large collec- 

 tion of organic remains has been made from the Durness lime- 

 stone, but it has not yet yielded materials for a satisfactory 

 stratigraphical correlation. The study of this collection, how- 

 ever, has confirmed and extended Salter's original sagacious 

 inference that the fauna of the Durness limestone shows a 

 marked North American facies, though, according to our present 

 terminology, we place this fauna in the Cambrian rather than in 

 the Silurian system. Below the Durness limestone lies the 

 dolomitic and calcareous shaly group known as the ' Fucoid 

 beds,' which, though crowded with worm-castings, has hitherto 

 proved singularly devoid of other recognizable organic remains. 

 In following this group southwards through the Dundonnell 

 Forest, in the west of Rosshire, my colleague, Mr. John Home, 

 found that, a few feet below where its upper limit is marked by 

 the persistent band of ' Serpulite grit,' it includes a zone of 

 blue or almost black shales. During a recent visit to him on 

 his ground, when he pointed out to me this remarkable zone, 

 I was struck with the singularly unaltered character of these 

 shales, and agreed with him that if fossils were to be looked for 

 anywhere among these ancient rocks, they should be found 

 here, and that the fossil- collector, Mr. Arthur Macconochie, 

 should be directed to search the locality with great care. The 

 following week this exhaustive search was undertaken, and Mr. 

 Macconochie was soon rewarded by the discovery of a number 

 of fragmentary fossils, among which Mr. B. N. Peach, who was 

 also stationed in the district, recognized what appeared to him 

 to be undoubtedly portions of Olenellus. The importance 

 of this discovery being obvious, the search was prosecuted 

 vigorously, until the fossiliferous band could not be followed 

 further without quarrying operations, which in that remote 

 and sparsely inhabited region could not be at that time 

 undertaken. The specimens were at once forwarded to me, 

 and were placed in the hands of Messrs. Sharman and 

 Newton, Palaeontologists of the Geological Survey, who con- 

 firmed the reference to Olenellus. More recently Mr. Peach 

 and Mr. Home, in a renewed examination of the ground, 

 have found, in another thin seam of black shale mterleaved in 

 the * Serpulite grit,' additional pieces of Olenellus, including a 

 fine head-shield with eyes complete. There may be more than one 

 species of this trilobite in these Rosshire shales. The specific de- 

 terminations and descriptions will shortly be given by Mr. Peach. 

 The detection of Olenellus among the rocks of the North-west 

 Highlands, and its association with the abundant Salterella of 

 the 'Serpulite grit,' afford valuable materials for comparison 

 with the oldest Palaeozoic rocks of other regions, particularly 

 of North America. The 'Fucoid beds' and ' Serpulite grit,' 

 which intervene between the quartzite below and the Durness 

 limestone above, are now demonstrated to belong to the lowest 

 part of the Cambrian system. The quartziles are shown to 

 form the arenaceous base of that system, while the Durness 

 limestones may be Middle or Upper Cambrian. On the other 

 hand, the Torridon sandstone, which Murchison placed in the 

 Cambrian series, can now be proved to be of still higher anti- 

 quity. The marked unconformability which intervenes between 

 it and the overlying quartzite points to a long interval having 

 elapsed between the deposition of the two discordant formations. 

 The Torridon sandstone must therefore be pre-Cambrian. 

 Among the 8000 or 10,000 feet of strata in this group of sand- 

 stones and conglomerates, there occur, especially towards the 

 base and the top, bands of grey and dark shales, so little altered 

 that they may be confidently expected somewhere to yield re- 

 cognizable fossils. Already my colleagues have detected traces 

 of annelids and some more obscure remains of other organisms 



NO. I 142, VOL. 44] 



