March 4, 1886] 



NA TURE 



417 



same black character, the lava broken in innumerable blocks, 

 and setting out in vivid colour the verdure of the river banks." 



A good deal of what has been said respecting the volcanic 

 district of Itasy also holds good in regard to that of the Betafo 

 valley and neighbourhood, where, however, the volcanic cones 

 are fewer, and where trachytic domes do not appear to exist. 

 One of the volcanoes in the Betafo valley, lavoko, is of greater 

 dimensions, and has a much larger crater than any to be found 

 about Itasy. From this volcano a large sheet of basaltic lava 

 has issued, upon which are to be found in abundance various 

 species of plants, notably a Euphorbia and a stonecrop (Kitch- 

 ingia). Almost all the plants growing on this lava-bed, how- 

 ever, are of a succulent character, and can dispense with soil, 

 requiring merely a foothold. On the sides of lavoko may 

 be picked up fragments of calcined gneiss, which have been torn 

 from the sides of the vent in t':e passage upward of the volcanic 

 matter. On some of the cones numerous crystals of augite as 

 large as marbles may be found among the volcanic debris. 

 There is one volcano, Tritriva, near Betafo, which, inasmuch as 

 it is different in character from any others mentioned above, 

 deserves a few words. It is one of those volcanoes off which 

 the summit has been blown by explosive action, leaving what is 

 known as a crater-ring, which is now the site of a small lake. 

 The lake is not more than loo or 2 o feet in diameter, perhaps 

 not as much as that ; but there is reason to suppose that it is of 

 very great depth. The inner sides are steep for the greater part 

 of the circumference, but on one side the lake is easily 

 accessible. 



It is possible that, when the country is more thoroughly 

 explored, it may be found that the volcanoes near Itasy and 

 those in the Betafo valley are connected by intermediate ones ; 

 indeed, on Dr. MuUens's map several craters are shown some- 

 what west of a jtraight line drawn between these two volcanic 

 districts. 



About 25 or 30 miles to the north-east of .Antananarivo I dis- 

 covered, a couple of years ago, several small volcanic craters. 

 These also seem to belong to the class of crater-rings or explosion 

 craters. Although fragments of volcanic matter have been 

 ejected from them, they are not in such quantity as to form a 

 cone; and the craters, none of which exceed 100 yards in dia- 

 meter, and 30 feet in depth, have been forjned probably by a 

 single explosion of the pent-up forces below. With the excep- 

 tion of scorite and lapilli, which are sparingly scattered about, 

 there is no visible sign of volcanoes, and one may come to the 

 very verge of the craters before being aware of their existence. 

 Two of the largest craters consist of saucer-shaped depressions, 

 but are rather elliptical than circular in form ; the others consist 

 mostly of small cavities, deep in proportion to their width. 

 Several of the craters are occupied by sheets of water, with 

 rushes and other aquatic plants growing around their margin. 



Besides the volcanic phenomena mentioned above, thermal 

 springs occur in various localities in the interior of Madagascar. 

 The following is an analysis by Dr. Parker of water from springs 

 in the district of Antsirabe : — 



"On evaporation, one pint {20 oz.) of water from each spring 

 yielded the following quantities of solid salts : — 



Spring No. r yielded 40 grs. of salt, or 2 gi's. to I 01. of water'. 

 ,, „ 2 „ 3S „ ,, l'9 ,, I „ 



,, ,, 3 .. 42 .. .. 2'i .. I .. 



,, ,, 4 ,1 28 ,1 .> I'4 >> I >> 1. 



All these springs contain the same ingredients, viz. lime, mag- 

 nesia, soda, and potash, in combination with chlorine, iodine, 

 sulphuric acid, and carbonic acid, with the addition of free 

 carbonic acid gas." 



At Antsirabe there is a deposit from one of these springs of 

 carbonate of lime, which is occasionally used for building pur- 

 poses in the capital. Brtbbles of carbonic acid may be seen 

 rising from the surface of the deposit, and at one point, where 

 there is a small spring, a mass of calc-sinter has been formed 

 which, speaking from memory, is pi-obably 12 feet high by 18 

 feet long. 



In one of the valleys in the vicinity of the cratei'-rings of 

 Ambohidratrimo, spoken of above, there is a deposit of siliceous 

 sinter. It appears in one or two places, scarcely rising above 

 the surface of the ground, in a valley of rice- fields, and has 

 been deposited by springs which have long since ceased to flow. 

 The sinter is exceedingly hai-d and compact, and is used by the 

 natives for fire-flints. In some portions of it numerous fossils 

 of a species of Equisetum are embedded. The longitudinal 



strice leave no doubt as to the nature of the plant. The nstular 

 stem has been filled in, and the vegetable substance entirely 

 replaced, by silex. The stems of some of these fossil plants are 

 quite half an inch in diameter. Now, the only Equisetum 

 found in Central Madagascar at the present time is E. ramosisf.- 

 iiiiim, but this never attains to such a thickness as the Equiseta 

 in the sinter ; so that the fossil species have become extinct 

 since the springs which deposited the geyserite were in a state 

 of activity. 



So little is known respecting earthquake phenomena in Mada- 

 gascar, no scientific observations ever Iraving Iieen instituted, 

 that it is scarcely worth while to refer to the subject. However, 

 it may be stated that scarcely a year passes without one or more 

 shocks being exper-ienced in Central Madagascar, though ^they 

 are never severe or of long duration ; and the destruction caused 

 by these earth-waves in some parts of the world is entirely 

 unknown here. The natives, I may say in passing, strangely 

 imagine that earthquakes are caused by a whale (Trozona) turning 

 on its back. 



Extinct volcanoes and thermal springs exist also in other parts 

 of the island, birt so httle is known about them that I can do 

 no more than merely alhtde to their existence. R. Baron 



Antananarivo, Madagascar, December 2, 1885 



Coal-Dust and E.xplosions 



Those who have given the labours and conclusions of workers 

 antecedent to, and contemporaneous with, Mr. W. Galloway, 

 on the subject of the part played by coal-dust in mine explo- 

 sions, the car-eful consideration which these merit in common 

 with the results and writings of that zealous exponent of the 

 question, will hardly feel disposed to concur in his conclusion 

 that, except by him, " the very simple, and yet all-important, 

 element " to which he refers in his recent letter has been treated 

 with neglect. 



On the other hand, they will consider that when Mr. Gallo- 

 way "goes the length of crediting coal-dust with the rSld of 

 principal agent (in coal-mine explosions), and of relegating fire- 

 damp to a secondary position," he altogether loses sight of some 

 very obvious facts which forbid so sweeping a conclusion. 



Any one who is led, by special interest in the subject, to study 

 the forthcoming Repot of the Royal Commission on Mine 

 Accidents, will find that the important part which may be, and 

 no doubt freqrtently is, taken by dust in coal-mine disasters is 

 recognised to its full extent, and that, in a careful consideration 

 of the accumulated knowledge on this subject, all due weight 

 has been given to the experimental results arrived at by Mr. 

 Galloway and others. Fredk. A. Abel 



March 3 



Deposits of the Nile Delta 



Permit me to say that Prof Juddis in error in supposing that 

 I intended to withdraw my statement that desert sand underlies 

 the Nde alluvium at a very moderate depth. The general 

 succession of the newer deposits of Lower Egypt, according to 

 the information I have been able to obtain (and which I have 

 endeavoured to state as plainly as possible) is as follows, in 

 descending order: (l) Modern alluvium, varying from zero to 

 about 40 feet, and of course more in old eroded channels. (2) 

 Desert sand of the Post-Glacial continental period. (3) Pleistocene 

 or Isthmian deposits, lacustrine, estuarine, or marine. The 

 question is not whether this succession exists— that I am prepared 

 to ar-gue on other grounds — but whether it appears in any or all 

 of the recent borings. It is scarcely necessary to say that such 

 general succession admits of alternations at the junctions of beds, 

 and of local absence of some of its members. On finding, how- 

 ever, that the recent borings had been stopped by quicksand at 

 the depth of about 35 feet, and that this quicksand consisted of 

 the rounded grains of desert sand, and was mixed with gray clay 

 or marl, and concretions like those of the Isthmian formation, I 

 naturally concluded that the succession above referred to was 

 distinctly indicated. Prof. Judd now affirms, as I understand, 

 that, in all the Delta borings, mud of "precisely similar mineral 

 character " to that of the surface extends to the bottom. The 

 evidence of this, as well as the promised consideration of the 

 other points to which I have alluded, I am content to wait for 

 till the report appears in full. J. Wm. Dawson 



Montreal, February 18 



