220 



HARDWICKE'S SCIENCE-GOSSIP. 



consist of diatoms, mixed with what I am told, by my 

 friend Mr. Siddall, are the chitinous and siliceous 

 skeletons and macrospores in great abundance of 

 Isoetes lacustris, Quillwort, in perfect preservation. 



On analysing the diatomaceous earth I find it to 

 contain the following constituents when dried at 

 ioo°C: 



PEAT 

 DIATOMACEOUS EARTH 



GRAVEL 



Fig. 135.— Diagrammatic Section of Llyn Arenig Bach. 



Silica 88-32 



Alumina 3'44 



Peroxide of iron 1*30 



Lime '13 



Magnesia '07 



Soda '28 



Water 6*42 



99-96 



It is very similar in composition to the deposit at 

 Cwm Bychan, which contains — 



Silica 84-50 



Peroxide of Iron and Alumina . . . 7*35 



Water 6-37 



Lime, Magnesia, &c 1*78 



xoo'oo 



The amount of silica contained in the Arenig earth 

 after all the water has been driven off is equal to 

 94-3 per cent., and the silica of the Cwm Bychan 

 earth equal to 90 per cent. 



The composition of the peat when dried at ioo° C. 

 until constant is — 



Organic Matter 60 



Ash or Mineral Matter 40 



Of this mineral matter 93 per cent, is insoluble in 

 hydrochloric acid, and consists almost entirely of 

 diatom. 



If we take the area of the lake at 594,000 square 

 feet, and the thickness of the deposit all over the 

 lake at 1 foot,* there will be 594,000 cubic feet of 

 the earth, and I find that one cubic inch of the dried 

 earth weighs 40 grains, so that the total weight of 

 the deposit will be equal to 2645 tons. Since writing 

 the above I have obtained some peat from the 

 bottom of a small lake, also in Merionethshire, called 

 Llyn Du. 



This peat is very rich in diatoms, and a large number 

 of the forms are similar to those occurring in the Arenig 

 deposit, but the remains of Isoetes are absent. 



I think from the number of diatoms present that 

 there is a diatomaceous deposit below the peat in this 



* The deposit is most probably thicker towards the centre of 

 the lake. 



lake, and I also think that these deposits may be 

 found in most of the lakes in Wales, when they come 

 to be thoroughly examined, and that they have not 

 yet been observed because they occur at a consider- 

 able distance below the surface, and may in many 

 cases be covered by a layer of peat. 



VOLCANIC CONES, THEIR STRUCTURE 

 AND MODE OF FORMATION. 



By H. J. Johnston-Lavis, F.G.S., &c. 



OUR general ideas of its appearance, if we have 

 never seen a volcano, differ considerably 

 from what we find, when actually brought in contact 

 with one. 



We always have the tendency to associate a 

 mountain as the site of volcanic outbursts. Such is 

 the case in general rule, though with many exceptions. 

 In fact, the variations are so great that in many cases 

 we should be inclined to attribute the extreme forms 

 to totally different origins, were there not existing 

 intermediate ones which demonstrate that they are 

 all varied modifications of one almost_uniform series 

 of physical effects. 



Thus to one looking first at the vast volcanic cone 

 of Cotopaxi, almost perfect in form, and comparing 

 it with the ring-like cavity of Astroni in the Phlegrean 

 field, it would be almost incomprehensible to believe 

 that these two extremes are the result of identical 

 forces acting much in the same manner and producing 

 such widely different effects. But in the latter district 

 we have not to travel far to find other vents that act 

 as interpreters in explaining these variations of forms. 

 In the present paper it will be my endeavour to 

 explain the building up of what we will call a normal 

 volcanic cone, and then afterwards to point out the 

 extreme variations to which such a mass is liable. 



Given a large volume of heated vapours and liquid 

 rock that has burst its way upwards through the sub- 

 jacent strata, in what way will it manifest its presence, 

 and what traces will it leave behind ? This vapour 

 does not seem to exist separately from the molten 

 rock or lava at any great depth, but as it approaches 

 the surface, the enormous pressure is reduced, the 

 water and other gaseous matter expand, separate 

 themselves into little bubbles scattered through the 

 highly-heated liquid magma. These will collect 

 together, to a certain extent, and from their lightness 

 will float to the surface of the lava and there burst. 

 The vapour may have commenced to form at great 

 depths and in its upward journey have become exceed- 

 ingly bulky, so when it reaches the surface it would 

 escape with a loud explosion. If we watch lava in 

 the crater of a volcano in a quiescent state, such as 

 Vesuvius, we see these great bubbles, so to speak, 

 continually forming and bursting. As they burst, the 

 surface of the vesicle is blown up as soft pasty frag- 



