/une 26, 1884 j 



NA TURE 



193 



have been made between the present sea-beds and the Chalk, the 

 Gault, and the Greensands, which appear to be among the deepest 

 water deposits now accessible as dry land. Instead of this we 

 are merely told that chalk "must be regarded as having been 

 laid down rather along the border of a continent than in a true 

 oceanic area" (Nature, p. 134). All geologists are aware, 

 since the publication of Dr. Gwyn Jeffreys' address to the British 

 Association, and the appearance of Mr. Wallace's "Island 

 Life," that some naturalists regard the Chalk as a shallow-water 

 formation, but the former opinion, pronounced as it was by one 

 of the most competent judges, was based exclusively on the 

 present habits of the very few genera of Mollusca that have sur- 

 vived from the Chalk period, and seems quite in contradiction to 

 the far more important groups, the Sponges, lichinodermata, 

 and the minute organisms of which the formation is so largely 

 composed, while no opinion has yet found its way into the hands 

 of geologists regarding the depth of water indicated by the 

 Crustacea and the fishes of the Chalk. Mr. Wallace's collation 

 of the Chalk, as a formation, with the decomposed coral mud of 

 Ooahu, is so fantastic as to have failed to carry conviction to 

 the mind of any competent geologist. The points of resem- 

 blance between some Globigerina ooze and the Chalk are so 

 numerous and peculiar, that surely the assertion that the latter 

 is a littoral formation, while the former is oceanic, requires strong 

 support. The relative analyses of chips from the Chalk and of 

 Globigerina ooze, quoted by Mr. Wallace, are not by any means 

 final or conclusive. We all know that the silica has been re- 

 moved and segregated into flints from the White Chalk at Shore- 

 ham, and that the iron and other metals are also segregated into 

 crystallised masses, so that a comparison of the Chalk, minus 

 these, is misleading. In like manner the Grey Chalk at Folke- 

 stone has lost all its oxide of iron by segregation and crystallisa- 

 tion, and many of the layers are cherty, and unduly rich in silica 

 obtained probably at the expense of other layers in which it is 

 now relatively scarce. During the ages that Chalk has been 

 elevated and has acted as a sponge for the collection of rain water, 

 who can say what other of its constituents may not have been 

 dissolved away or metamorphosed ? Siliceous sponge skeletons 

 have been replaced by calcite, calcite shells have been replaced by 

 silica, whilst aragonite shells have been entirely dissolved away. 

 In like manner, can it possibly be contended that the absence of 

 volcanic matter in the Chalk is an important distinction between 

 it and Atlantic ooze ? It is an accidental lithological dis- 

 tinction, but nothing more, and merely shows that volcanic dust 

 was not being ejected in the same masses as at present. The 

 Cretaceous and Eocene eruptions, so far as I am aware, are all 

 fissure eruptions of vast magnitude, and the contemporary 

 rocks in their vicinity seem to show that they were not accom- 

 panied by the showers of ash that mark eruptions from craters 

 at the present day. Messrs. Renard and Murray have had ex- 

 ceptional opportunities of studying this question, and have no 

 doubt convincing proofs of their statement regarding the littoral 

 character of the Chalk deposit ; but I really think that, consi- 

 dering the national character of the undertaking which made 

 the collection of proof possible, it should no longer be withheld. 

 Geologists at present, supposing my feelings are generally 

 shared, are asked to believe that an enormous formation, which 

 shows little, if any, trace of the proximity of land, and abounds 

 with the remains of deep-sea life, was laid down upon a coast- 

 line ; but beyond the extravagant assertion that it is decomposed 

 coral-mud no reasons whatever for this belief are brought for- 

 ward, nor are any areas pointed out in which an equivalent to 

 the Chalk is in course of deposition. I cannot conceive why 

 our official geologists have ignored this, one of the most important 

 questions in the whole range of the science. It is little to our 

 national credit that, having spent vast sums in the collection of 

 evidence, we are still in the dark as to its geological significance. 

 J. Starkie Gardner 



A Rhyolitic Rock from Lake Tanganyika 

 The interesting note by Dr. H. J. Johnston- Lavis on a volcanic 

 rock from the shores of Lake Nyassa (Nature, p. 62) calls to my 

 mind a couple of specimens in my collection which, with not a 

 few others of interest, have perforce remained for some time un- 

 described. They were given to me by N. F. Robarts, Esq., 

 F.G. S., whc received them from Capt. Hore of the London 

 Missionary Society, by whom they were collected at Cameron's 

 Bay on the south-west of Lake Targanyika, a little north of the 

 Lofu River. As they are evidently fragments of the same kind of 

 rock, I have only had a slide prepared from one of them. The rock 



is externally of a pale yellowish- to reddish-gray colour ; compact, 

 but exhibiting faint traces of a fluidal structure, with occasional 

 spots resembling small crystals of decomposed felspar. A fresh 

 broken surface, however, shows the real colour to be a purplish 

 brown, streaked and mottled with a pale reddish tint. Micro- 

 scopic examination shows that the rock is a rhyolite, somewhat 

 darkened with numerous specks of disseminated ferrite, with 

 many clearer bands, indicative of a fluidal structure. In this 

 matrix are scattered crystals of decomposed felspar, not exceed- 

 ing 'i inch in diameter, and a few plates of a ferruginous mica, 

 also exhibiting signs of decomposition, with two or three 

 granules of quartz. With crossed Nicols a minute devitrification 

 structure is exhibited by the slide as a whole, and this is coarser 

 and stronger in the clearer bands. Here crystalline quartz is 

 developed, which assumes with the felspars on occasion a 

 spherulitic or sometimes approximately micrographitic structure. 

 The larger felspar crystals are rather decomposed, but ortho- 

 clase and a plagioclastic felspar can be recognised. Mam 

 distinct granules of iron peroxide (? haematite) are scattered 

 about. Examination with high powers causes me to doubt 

 whether the devitrification is complete in all parts of the slide, 

 and whether the phenomena are not rather due to the develop- 

 ment of a large number of minute crystallites of not very regular 

 form in an isotropic base. In this, however, there is nothing 

 exceptional. 



From the structure I should consider it more probable thai 

 the specimen had been taken from a flow than from a dyke. 1 

 should suspect the devitrification structure to be the result ol 

 secondary change, and the rock not a very modern one. In 

 some respects it reminds me of the pre-Cambrian rhyolites (de- 

 vitrified) of Britain, but I should think it had not quite so high 

 a percentage of silica, i e. that this did not exceed 70, and per- 

 haps was rather less. Among the rhyolitic rocks which 1 

 described from Socotra (Phil. Trans. 1SS3, p. 273), collected 

 by Prof. I. B. Balfour, were some of a rather dark purple colour, 

 not unlike to this specimen from Lake Tanganyika. 



T. G. Bonney 



Aseismatic Tables for Mitigating Earthquake Shocks 



In Mr. Topley's paper on the Colchester earthquake, which 

 appeared in Nature, vol. xxx. p. 60, he mentions the aseismatic 

 joint designed by my father, Mr. David Stevenson, for mitigating 

 the effects of shocks on lighthouses in countries subject to earth- 

 quakes, and from information which Mr. Topley has received and 

 cites it would appear that the appliance had been tried in Japan, 

 found wanting, and abandoned. The facts of the case, however, 

 are as follows, and are supplied to me by Mr. Simpkins, w ho 

 was engaged in fitting up the apparatus sent out from here, and 

 has only recently returned from Japan. Of the seven lighthouse 

 apparatus designed by Messrs. D. and T. Stevenson and furnished 

 with the aseismatic joint and sent out to Japan, there are three 

 at present in action, and have been so for ten years, viz. Miko- 

 moto, Siwomisaki, and Yesaki. At Iwosima and Satanomisaki, 

 in the south end of the island, the tables are screwed up so as not 

 to act, as it is reported that no earthquakes are felt at these 

 stations. At Tsuragisaki and Kashmosaki, which are revolving 

 lights, the steadying screws sent out with the apparatus (to pre- 

 vent the table oscillating while winding up the machine, which 

 is the main inconvenience felt, and which was foreseen) were for 

 some reason not put in at these stations, and the tables were 

 firmly strutted with timber to prevent any motion. These two 

 are the only lighthouses at which any damage has been done : 

 while those stations at which the tables are in operation hav< 

 never suffered at all, although they have been repeatedly sub 

 jected to shocks. 



With regard to the effect of wind, to which Mr. Topley alludes. 

 I may say that none of the towers are placed on tables, it is only 

 the apparatus inside the lantern which is so treated, although my 

 father proposed it for the towers themselves, and I have no doubt 

 that, from the experiments I saw made here, they would have 

 been equally effective. Two towers fitted with the tables were 

 made and sent out to Japan, but were unfortunately lost at sea 

 and not replaced. Charles A. Stevenson 



45, Melville Street, Edinburgh, June 16 



The "Cotton-Spinner" 



On seeing my article on this rare British Holothurian, Mrs. 

 Fisher — who, as Miss Arabella B. Buckley, is well known to a 



