Nov. 19, 1874J 



NATURE 



59 



The order of crystallisation of the component minerals was 

 shown to be the following :— Magnetite, felspar in large or small 

 distinct ciyscals, augite, felspathic or leucitic solvent. Some of 

 the first-fonned crystals were broken and rendered imperfect 

 before the viscid state of igneous fusion ceased. Even in such 

 modern lava-flows as that of the Solfatara considerable changes 

 had taken place by alteration and the replacement of one mineral 

 by another, and is ver>- generally in successive layers correspond- 

 ing to the crystal outlines. The frequent circular arrangement 

 of the glass and stone cavities near the circumference of the 

 minute leucite crystals in the lava of 1631 was thought to point 

 to the fact that after the other minerals had separated from the 

 leucitic solvent, the latter began to crystallise at numerous adja- 

 cent points ; and as these points approached one anoilier, solidi- 

 fication proceeded more rapidly, and these cavities were more 

 generally imprisoned than at the earlier stages of crystallisation. 

 In the example of the lava of 1 794, where the leucite crystals 

 were further apart, this peculiar arrangement of cavities was 

 almost unknown. The third part of the paper dealt with the 

 lavas and ashes of North Wales ; and the author thought that 

 the following points were established :— I. Specimens of lava 

 from the Arans, the Arenigs, and Snowdon and its neighbour- 

 hood, all have the same microscopic structure. 2. Tliis struc- 

 ture presents a hazy or milky-looking base, with scattered par- 

 ticles of a light-green dichroic mineral (chlorite), and generally 

 some porphyritically imbedded felspar crystals or fragments of 

 such, both orthoclase and plagioclase. In polarised light, on 

 crossing the Nicols, the base breaks up mto an irregular- 

 coloured breccia, the colours changing to their complementaries 

 on rotating either of the prisms. 3. Finely bedded ash, when 

 highly altered, is in some cases undistinguishable in microscopic 

 structure from undoubted felstone. 4. Ash of a coarser nature, 

 when highly altered, is also very frequently not to be distin- 

 guished from felstone, though now and then the outlines of some 

 of the fragments will reveal its true nature. 5. '1 he fragments 

 which make up the coarser ash-rocks seem generally to consist of 

 felstone, containing both orthoclase and plagiocase crystals 

 or fragments ; but occasionally there occur pieces of a more 

 crystalline nature, with minute acicular prisms and pla- 

 gioclase felspar. 6. In many cases the only tests that can be 

 applied to distinguish between highly altered ash-rock and a fel- 

 stone are the presence of a bedded or fragmentary appearance 

 on weathered surfaces, and the gradual passage into less altered 

 and unmistakable ash. In the fourth division of his paper the 

 author described some of the lavas and ashes of Cumberland of 

 Lower Silurian age. With regard to these ancient lavas, the 

 following was given as a general definition :— The rock is gene- 

 rally of some shade of blue or dark green, generally weathering 

 white round the edges, but to a very slight depth, it frequently 

 assumes a tabular structure, the tabulce being often curved, and 

 breaks with a sharp conchoidal and flmty fracture. Silica, 59-61 

 per cent. Matrix generally crystalline, containing crystals of 

 labradorite or oligoclase and orthoclase, porphyritically im- 

 bedded, round which the small crystalline needles seem fre- 

 quently to have flowed ; magnetite generally abundant, and 

 augite tolerably so, though usually changed inio a soft dark-green 

 mineral ; apatite and perhaps olivine as occasional constituents. 

 OecasioitaUy the crystalline base is partly obscured and a felsitic 

 structure takes its place. The Cumberland Uvas were shown to 

 resemble the Solfatara greystone in the frequent flow of the 

 crystalline base, and the modern lavas generally in the order in 

 which the variour minerals crystallised out. In external struc- 

 ture they have, for the most part, much more of a felsitic than a 

 basaltic appearance. In internal structure they have considerable 

 analogies with the basalts. In chemical composition they are 

 neither true basalts nor tiue felstones. In petrological struciure 

 they have much the general character of the modern Vesuvian 

 lavas ; the separate flows being usually of no great thickness, 

 being slaggy, vesicular, or brecciated at top and bottom, and 

 having often a considerable range, as if they had flowed in some 

 cases for several miles Irom their point of eruption. Their gene- 

 ral microscopic appearance is also very different from that of 

 such old basalts as those of South Stafford and some of those of 

 Carbuniferous age in Scotland. On the whole, while believing 

 that in some cases the lavas in question were true basalts, the 

 author was inclined to regard most of them as occupying an 

 intermediate place between felsitic and doleritic lavas ; and as 

 the felstone-lavas were once probably trachytes, these old 

 Cumbrian rocks might perhaps be called Felsidolerites, answering 

 in position to the modern Trachy-dolerites. A detailed examina- 

 tion of Cumbrian ash-rocks had convinced the author that in I 



many cases most intense metamorphisni had taken place that 

 the finer ashy material had been partially melted down and a 

 kind of streaky flow caused around the larger fragments. ' There 

 was every transition from an ash-rock in which a bedded or 

 fragmentary structure was clearly visible, to an exceedingly close 

 and flinty felstone-like rock, undistinguishable in hand specimens 

 from a true contemporaneous trap. Such altered rocks were 

 however, quite distinct in microscopic structure from the 

 undoubted lava-flows of the same district, anil olten distinct also 

 from the Welsh felstones, although some were almost identical 

 microscopically with the highly altered ashes of Wales, and 

 together with them resembled the felstone-lavas of the same 

 country. This metamorphism among the Cumbrian rocks 

 increases in amount as the great granitic centres are approached • 

 and it was believed by the author that it took place mainly at 

 the commencement of the Old Red period, when the rocks in 

 question must have been buried many thousands of feet deep 

 beneath the Upper Silurian strata, and when probably the Esk- 

 dale granite was formed, perhaps partly by the extreme meta- 

 morphism of the volcanic series during upheaval and contortion. 

 The author stated his belief that the Cumbrian volcanoes were 

 mainly subaerial, since some 12,0:0 ft. of ash- and lava-beds 

 had been accumulated without any admixture of ordinary sede 

 mentary material, except quite at the base, containing scarcely 

 any conglomeratic beds, and destitute of fossils. He believed also 

 that one of the chief volcanic centres of tie district had been the 

 present site of Kenwick, the low craggy hill called Castle Head 

 representing the denuded stump or plug of an old volcano. The 

 author believed that one other truth of no slight importance 

 might be gathered from these investigations, viz., that neither 

 the careful inspection of hand specimens nor the microscopic 

 examination of thm slices would in all cases enable truthful 

 results to be arrived at, in discriminating between trap and 

 altered ash-rocks ; but these methods and that of chemical 

 analysis must be accompanied by oftentimes a laborious and 

 detailed survey of the rocks in the open country, the various beds 

 being traced out one by one and their weathered surfaces parti- 

 cularly noticed. 



Physical Society, Nov. 7.— Prof. W. G. Adams, F.R.S., 

 in the chair.— A paper by Mr. G. F. Rodwell was read, on an 

 instrument for multiplying small motions. It consists of a train 

 of multiplying wheels, the first of which is moved by the bar 

 whose elongation is to be measured, while the teeth of the last 

 engage witn the threads of an endless screw whose axis is ver- 

 tical, and carries at its extremity a long index moving over a 

 graduated circle. The multiplying power of the instrument is 

 very great ; its defects are its want of steadiness, great internal 

 strain, and the difficulty of bringing the index back to zero when 

 the pressure on the lever connected with the first wheel is re- 

 moved.— Prof. Foster, F.R.S., made a communication on the 

 geometrical treatment of certain elementary electrical problems. 

 The object of this communication ;was to illustrate the facility 

 and clearness by which certain of the electrical problems occur- 

 ring in elementary instruction could be treated by easy geometri- 

 cal m.ethods. Its application was shown in the following cases : 

 The calculatioa of the quantity of heat evolved in a galvanic • 

 circuit ; the calculation of the electromotive foice and of the per- 

 manent resistance of a voltaic battery from two deflections of a 

 tangent-galvanometer ; the determination of the joint resistance of 

 several conductors combined in multiple-arc ; and the deter- 

 mination of the distribution of potential and strength of the 

 currents formed by connecting the similar poles of two unequal 

 batteries with the opposite ends of the same conductor. — Prof. 

 Guthrie read a paper on salt solutions and water of crystallisa- 

 tion. The absorption of heat which occurs when a sa't is dis- 

 solved in a liquid was shown to depend not only on the relative 

 specific heats of the salt and the liquid, bat also on the molecular 

 ratio of the resulting solution. This ratio declared itself optically 

 (i) by the singularity of the refractive index when the critical ratio 

 was obtained, (2) by the singularity of density at the same point, 

 (3) by the heat absorbed when (a) a saturated solution was mixed 

 with the medium, and (ff) when the salt ilseli was dissolved in a cer- 

 tain quantity of the medium. The coadition of maximum density 

 of wa'.er was referred to the existence of a definite hydrate of water. 

 It was shown that every salt soluble in water was capable of 

 uniting with water in a definite ratio (by weight), forming definite 

 solid compounds of distinct crystalline form and constant melting 

 and solidifying points. It was supposed that the ratios of sucli 

 union are not incommensurable with the ratios of chemical 

 weight, and that the new class of bodies which only exist below 



