470 



NATURE 



\_Sept. II, 1879 



the beds in relation to tbetriassic system, and remarking on the 

 absence of the upper mottled sandstone, as well as the 

 «' Muschelkalk," in this part of England, the author described 

 the various strata of the district, as shown on the line between 

 Retford and Gainsborough, and pointed out the want of any 

 division in the beds of the lower kenper sandstone, as in other 

 localities, and the absence of any boundary line between this 

 series and the " red marls " above. 



On a Northerly Extension of the Rhatic Beds at Gainsborough, 

 by F. M. Burton, F.G.S. — At the meeting of the British As- 

 sociation at Nottingham in 1866, the author announced the 

 discovery of beds of the rhjetic age at Gainsborough, a full 

 account of which will be found in the Quarterly yournal of the 

 Geological Society for 1867. These beds occur to the south of 

 Gainsborough, on the Great Northern line between Doncaster 

 and Lincoln, and were discovered through the lowering of the 

 gradients of that line in 1S66. The author has since found them 

 in a cutting of the Manchester, Sheffield, and Lincolnshire 

 Railway at BIyton, about five miles to the north of Gainsborough, 

 where they must have been exposed since the making of that line in 

 the year 1848, though hitherto they have remained unrecorded. 



The Age of the Penine Chain, by E. Wilson, F.G.S. — In this 

 paper the author combated the generally accepted view of the 

 post-Permian origin of the Penine chain, and contended for a 

 pre-Permian upheaval. In support of this opinion the following 

 facts were cited : The Yorkshire coal-basin was admittedly pre- 

 Permian, for north of Nottingham the magnesian limestone 

 everywhere overlaps the coal measures ; but the axis of this basin 

 is parallel with the Penine chain, and was evidently determined 

 by the same series of movements that upraised that chain. The 

 Permians disappear on the west in approaching the Penine chain ; 

 in this direction also the marl slates attenuate, and these and 

 the magnesian limestone become more sedimentary, as if ap- 

 proaching a margin. Mountain limestone pebbles occur in 

 Permian breccias on one or both sides of the Penine axis. Many 

 fragments of carboniferous rocks occur in lower Bunter sand- 

 stone (breccias) on the borders of Notts and Derbyshire ; but 

 the author finds no fragments of Permian rocks in these breccias. 

 No outliers of Permian rocks are found at any distance west of 

 the magnesian limestone escarpment between Nottingham and 

 Northumberland. The character and succession of the Permians 

 on the two sides of the Penine chain are very dissimilar. 



On Geological Episodes, by J. F. Blake, M.A., F.G.S.— 

 Geological nomenclature was first founded on the theory of 

 universal deposits ; then the idea of lateral changes was intro- 

 duced, with the necessary misuse of lithologically descriptive 

 names ; ultimately all deposits were seen to have their boundaries. 

 Beds deposited in distinct areas can thus be proved only homo- 

 taxial, and these are by no means necessarily synchronous. The 

 object of this paper is to show that a somewhat similar principle 

 ought to govern a!l our geological classification. A single area is 

 defined to be one over which we can trace one or more related form- 

 ations consecutively, and which formations contain identical char- 

 acteristic fossils. Deposits in single areas may be compared as to 

 time and divided into life zones ; but these in different areas are 

 homotaxial only. In each single area the outlines and characters 

 of the several deposits must first be determined and denoted 

 accordingly. 



In studying any group of rocks in a single area it is seen that 

 some members have a much wider range than others. Such 

 differences in range are accompanied by marked differences in 

 character and point to differences in the circum';tances of depo; it. 

 The Jwide-spread formation indicates uniform changes of 

 level over the area and a mixture of deposits — such circum- 

 stances may be called normal. But mere local changes may 

 bring more restricted areas into peculiar physical conditions. 

 Such local changes maybe called "geological episodes," and 

 they will result in the formation of deposits of marked character 

 easily distinguishable from the normal. 



The first point is to determine the characters by which an 

 episcjdal deposit may be differentiated from a normal one. The 

 supreme test is that derived from its definition, i.e. its local de- 

 velopment ; but if it be very small, it may be insignificant ; if 

 relatively very large, the distinction may be of no consequence. 

 As a rule argillaceous rocks are normal, and arenaceous and 

 calcareous episodal ; but this is by no means universal. When 

 the normal formation of a period is determined, the episodes are 

 marked by their differing mineral nature. The two kinds of 

 deposits may also be determined by the nature of their fossils, 

 after we have first discovered what kinds of fossils are usually 



episodal. For this purpose those fossils which are found in all 

 kinds of rocks, and therefore appear to have been indifferent as 

 to their physical surroundings, may be called invariant, and 

 those found only under particular conditions, and which change 

 their locality as these conditions change, covariant. Invariants 

 only are suited for zonal classification ; covariants are cliaracter- 

 istics of episodes. A table is drawn up showing the classes, 

 families, and genera which may be covariant, according to the 

 imperfect observations of the past. The chief covariants are a 

 few foraminifera — the sponges — a large number of hydrozoa 

 and actinozoa, some crinoids, the blastoids, a few lamellibranchs, 

 and at least half the gasteropod families. 



The main proposition is that similar, but distinct episodes, in a 

 normal series of strata are neither necessarily nor probably of the 

 same age. The true method of geological classification is there- 

 fore to arrange only the normal deposits in a series by their 

 stratigraphy and their invariant fossils, while the episodes are 

 put in their place as such. 



These doctrines applied to British strata yield'the following 

 results : No episodes are recognised in Cambrian or pre-Cam- 

 brian rocks. In the lower Silurian, the Durness limestone, the 

 Llandeilo flags, the Bala limestone and the Caradoc sandstone, 

 and the May Hill and Llandovery beds are characterised as such. 

 Hence the term " Caradoc " is inapplicable as a name for the 

 normal portion of the reries. The "Colonies" of Barrandc 

 may be episodes recurrent on the same area. In the upper 

 Silurian, the Wenlock and Aymestry limestone, the Denbigh 

 grits, and tilestones are episodes. The carboniferous series 

 present us with the Coomhola grits, Burdie House limestone. 

 Millstone, and Pennant grits, while the mountain limestone is 

 merely a gradually changing normal deposit. The episodes of 

 the Permian are the fossiliferous limestone and underlying mar) 

 slate. The absence of the Muschelkalk from England is not 

 regarded as due to its being an episode, but to our deposits as a 

 whole being formed in a distinct area, the true episodes of the 

 period being the Hallstadt, St. Cassian, and Dachstein beds. 

 The lias is remarkable for its great freedom from episodes, 

 which accounts for the success of its zonal classification, the 

 only exceptions being the Sutton series, and some of the middle 

 lias rock beds. The lower oolites, on the contrary, are almost 

 entirely episodal, none of the beds having a wide range. The 

 Yorkshire deposits were formed in a distinct area, and may cover 

 the period of the great oolite as well as the inferior oolite, 

 the deposits supposed to connect them with the latter being 

 episodes. The rocks above the Cornbrash formed one connected 

 series, as recognised by all German writers and some French, in 

 which the Kelloway rock, the Corallian, and the Portland rocks 

 are well marked episodes in this country. It is therefore sug- 

 gusted that the term "middle oolites" should be abolished from 

 the classification of British strata, and the whole be known as 

 upper oolites. The various episodes in this series on the Con- 

 tinent and in England will never be truly located until their real 

 character is seen, and it has been by the study of these rocks 

 that the doctrine of episodes has been suggested. 



In the cretaceous series the wea'den, the Tealby series, 

 and parts of the lower greensand are episodal, the ironsands 

 being the nearest approach to a normal formation. The upper 

 greensands are also episodes ; but the chalk, though calcareous, 

 is normal. 



The lower tertiaries, like the lower oolites, scarcely present 

 any normal deposits, the London clay being, though argillaceous, 

 episodal in character. 



In the result, the series of sedimentary rocks should be repre- 

 sented not by so many parallel lines, but in many cases by lenticular 

 masses, whose age is denoted by their position — according to a 

 table which presents their true character. It is urged, therefore, 

 tliat the names proposed — or else some better — be used to dis- 

 tinguish the different kinds of strata and fossils, in order to give 

 definition and importance to truths which must have long been 

 floating in the minds of geologists. 



The Surface Rocks of Syria, by J. Perry.— The paper was 

 suggested by an examination of the sandstone quarries at Baal- 

 bec. The rock is composed of a mixture of the particles of 

 limestone from the coast, and drift-sand. The mixture is con- 

 solidated layer by layer, and fresh rock of the same nature is 

 now in process of formation. The author explained how the 

 consolidation is produced by water dissolving the particles of 

 carbonate of lime and by alternations of temperature. The 

 author then gave some explanation of the veined and apparently 

 cracked appearance of certain limestones. 



