448 



NATURE 



{_Sept. 4, 1879 



have very small capacity, and which should yet be very sensitive, 

 the author has arranged the quadrants as pieces of a flat disk, 

 only I inch in diameter, and the needle has been bent round 

 them so as to be acted on by both their upper and lower surfaces 

 and their outside edge. 



4. A new rapid commutator. , 



This was invented by Prof. Cornu, of the Ecole Polytechnique, 

 Paris, who had the great kindness to devise it for the author of 

 this paper, who, when M. Cornu took up the matter, had just 

 constructed three different instruments for the experiments for 

 which this one is intended, all of which had proved unsuccessful. 



Some preliminary experiments with M. Cornu's instrument 

 have shown that it promises to be entirely satisfactory. It can 

 be used with either the large or small induction balance on the 

 one hand, and with a Holtz machine or battery of 500 or more 

 cells on the other. It reverses the electrification of the plates of 

 the balance eighteen times per second, and between each reversal, 

 short circuits, and puts to earth both poles of the induction 

 balance and both poles of the battery. By altering two screws 

 it can be arranged to short circuit and put to earth the poles of 

 the induction balance only, and to insulate the battery poles. 



5. Driving-wheel for the Cornu commutator. 



All the instruments have been constructed by Mr. Kieser, of 

 the firm of Elliott Brothers. 



SECTION C 



GEOLOGY 



Opening Address by Prof. P. Martin Duncan, F.R.S., 

 Vice-President of the Geological Society, Presi- 

 dent OF THE Section 



Everyone who is interested in the science which is especially 

 considered in this Section of the British Association for the 

 Advancement of Science must be impressed with the importance 

 of the geological construction of this district in determining its 

 physical geography, in producing the features of its landscapes, 

 and in originating and developing many of the industries of the 

 busy town of Sheffield. 



It was inevitable that you should be addressed, at the commence- 

 ment of your labours, upon the subject of the carboniferous 

 formation, especially as the intention of this peripatetic congress 

 is to advance science amongst tho?e who require it. It will 

 therefore be my privilege to bring before you some of the more 

 important generalisations of the day, and some other considera- 

 tions regarding the great formation which is so fully developed 

 in this part of England ; trusting that whilst many of you will 

 submit to be reminded of the results of the labours of the men 

 who have established our science and of those of yourselves, 

 some who desire further information than they have hitherto 

 obtained may be advanced in knowledge. 



Of all geological formations, the carboniferous is the most 

 important to mankind at the present time, and the most interest- 

 ing to the student. It gives the earliest clear and definite idea 

 of a land surface on the earth, or rather of the existence of 

 many lands ; for they are to be traced here and there from high 

 up in Arctic latitudes to Australia, and from the West of 

 America to Eastern Asia. It offers evidence of the existence, 

 even in those remote days as in the much later miocene age, of 

 astronomical conditions which do not now prevail. It yields 

 proofs of the persistence of a vast lowland flora during extra- 

 ordinary vicissitudes of the relative level of land and sea, and of 

 the existence of a fauna remarkable for its great fish and 

 amphibia, and whose air breathing mollusca and insecta are of 

 surpassing interest, forshadowing as they do many recent forms. 

 And its study indicates that the movements of the crusts 

 of the earth, which occurred during and terminated the 

 age, were of the grandest kind, and have been of the greatest 

 importance to mankind, destroying, it is true, all the vestiges of 

 a large part of a volume of the earth's history, but bringing coal 

 within the reach of the explorer and miner. 



This world-wide formation, usually very thick everywhere, 

 has all the evidences of having lasted during a vast age, and 

 there are present in it the relics of sea floors, of shallow seas 

 and estuaries, of land siurfaces, rivers, and marshes. The 

 volcanic activity of the age was great, and is capable of 

 demonstration. 



So deep are some of the sediments composing the carboniferous 

 formation in different parts of the world, that the idea of exact 

 contemporaneity is not necessarily much modified. It was in 



all probability " coal time " universally, and for a long duration. 

 But the beginning of the period was not synchronous in different 

 parts of the earth, neither was the ending. The Devonian age 

 lasted longer in some parts of the earth than in others ; and the 

 crust movements which so altered the physical geography of the 

 carboniferous hills, dales, and swamps as to develop a new 

 aspect of nature, terminated the period sooner in some quarters 

 of the globe than in others. In such a locality, however as 

 Eastern Hindostan, the duration of a carboniferous type into 

 the secondary ages is apparent. Hence, in spite of a recognised 

 general contemporaneity, it must be credited that carboniferous, 

 Devonian, Permian, and later deposits were accumulated early 

 and late during the lapse of one great age in distant parts of 

 the globe. 



The duration of the carboniferous age in the broadest sense, 

 may be attempted, but with no great success, to be estimated by 

 the time which must have elapsed during the world-wide disper- 

 sion of identical species ; and its biological relation to the 

 preceding and subsequent formations may be appreciated 

 from the fact that the carboniferous flora, lasting as it did from 

 the bottom to the top of the formation, was foreshadowed in the 

 Devonian, and that it founded the mesozoic. Thus the 

 Australian, Himalayan, British and North and South American 

 marine strata of the carboniferous age contain many identical 

 species of Brachiopoda — the variation from the English types, 

 which were the first described, being very slight. Amongst the 

 corals some forms are equally widely diffused. Now, according 

 to what occurs in nature at the present time, the movements of 

 species from one locality to another by ova, or by wafting of 

 the young — the only method of the lateral or horizontal pro- 

 gress of the brachiopoda — for instance, is impeded by many 

 physical conditions, and is constantly rendered abortive by 

 predaceous and obstructive living forms, and by what is called 

 the strugggle for existence. Migration, or rather the extension 

 of the locality of the species, for the first term implies much 

 more than was or is ever done, is so rarely possible to any great 

 extent under the present complicated natural history and physical 

 condition of the earth that the mind fails to grasp the time 

 which would lapse between the commencement of the dispersive 

 process and the establishment of identical species, even a few 

 thousands of miles off. To bring the subject a little nearer, 

 however, it is necessary to consider that the Arctic and 

 Antarctic cold areas and the frigid bathymetrical ocean zones 

 did not then exist, and that the movements of the crust, pro- 

 ducing extension of coast lines, were exceedingly frequent 

 during the age, and must have facilitated the dispersion of littoral 

 and moderately deep sea species. 



The dispersion of the species of the numerous crypto- 

 gamous plants was doubtless rapid in relation to that of the 

 animals, for their spores could be wafted to a great distance 

 by wind, and they do not appear to have had much to struggle 

 against. With the conifers it was different, and the examination 

 of the methods in which fir trees spread in favourable localities, 

 at the present time is very suggestive of exceeding slowness of 

 dispersion. Nevertheless, the cones of the conifera; were carried gl 

 here and there by water during the carboniferous age. II 



To add to the notion of the long duration of the age it must 

 be remembered that a succession of identical floras occurred 

 nearly on the same areas, involving repetitions of growth and of 

 migration. 



The growing of the vegetation of each swamp and lowland 

 tract, its accumulation and covering up with' sand, shales, and 

 gravel, occupied much time, and the last process involved the 

 destruction of considerable breadths of plant life. The forma- 

 tion of under-clay or warp, if the similar occurrences of the 

 present day be taken as examples, occupied much time, and then 

 a lapse occun-ed whilst the nearest flora supplied a new 

 vegetation to the virgin soil. 



In some instances the recurrence of vegetation was evidently 

 the result of spreading from no great distance ; but in others so 

 great a depth of sediment separates the consecutive deposits of 

 coal, and the great subsidence which took place is so evident, 

 that the migration must have been from a considerable distance, 

 and must have occupied commensurate time. In endeavouring 

 to appreciate this lapse of time, it must be remembered that, even 

 on the small surface of the United Kingdom, there was land on 

 some parts during the whole of the carboniferous age notwith- 

 standing the diversity of the deposits and the frequent occurrence 

 of marine conditions. 



It would appear that prior to those movements of the earth's 



