472 



NATURE 



{Sept. 15, 1 88 1 



circulate freely to far greater depths. Unfortunately, very little 

 i ; known of the substrata of volcanoes. Etna and Hecla ap- 

 patently stand on permeable Tertiary strata, Vesuvius on 

 Tertiary and Cretaceous sti-ata, while in South Auierija some 

 of the volcanoes are seemingly situated amongst palaaozoic and 

 crystalline rocks. Under ordinary circumstances all the per- 

 meable strata and all fissm-ed rocks become charged with water 

 up to the level of the lowest point of escape on the surface, or 

 if there should be an e.^cape in the sea-bed, then to that level, 

 plus a difference caused by friction. The e.\ti-eme porosity of 

 lavas is well known. All the water falling on the surface of 

 Etna and Ve-uvius (except ^^•here the rocks are decomposed and 

 a surface soil formed) di-appears at once, passing into the 

 fissures and cavities formed by the contraction of the lava in 

 cooling. Not only are these fissures filled, but the water lodges 

 in the main duct itself, and occasionally rises to a height to fill 

 the crater. Beue.ith the mass of fragmentary and cavernou, 

 volcanic materials forming the volcano, lies the original com- 

 pact mass of sedimentai7 strata, &c. Owing to the fortunate 

 circumstance of an Artesian well having been sunk at Naples, 

 we know the underlying sedimentary strata there to consist of 

 alternating strata of marl, sands, and sandstones, some water- 

 bv-'aring, others impermeable. The water from the lowest spring 

 reached in this boring rose at first 8 feet above the surface, and 

 Si feet above the sea-level. Where the strata crop out in the 

 sea-bed, the same pressure of the column of inland water forces 

 the fre-h water outwards so as to form a freshwater spring in 

 the sea, as at Spezzia and elsewhere on the Mediterranean coast. It 

 is this fundamental hydrostatic principle which keeps wells in 

 islands, and in shores adjacent to the sea, free from salt water, as 

 in the I le of Thanet. Where, however, the he.id of inland waters 

 is small or impeded, sea -water w ill enter the permeable strata, and 

 spoil the springs, as in the case of the Lower Tertiary sands at 

 Ostend, and the Lower Green-and at Calais and in the Somme, in 

 which latter department the underground spring was found affected 

 to a distance of about one mile from the sea, but pure at a di-tance 

 of nine miles. Further, if where the head of inland water is 

 sufficient to force back the sea-water under ordinary conditions, 

 those ordinary conditions are disturbed by pumping to an extent 

 that lower- the line of water-level to bebw that of the sea-level, 

 then the sea- water will flow inward > until an equilibrium is esta- 

 blished. The Adw of water under a volcanic mountain may be 

 also influenced by the quaquaversal dip, which there is some 

 evidence that the underlying strata there take, owing probably to 

 the removal of matter from below, and the v\eight of the moun- 

 tain. If we are to a-su ne that the volcanic a-hes and tufas below 

 Naples are subaerial, tlie original land-surface has sunk not less 

 than 665 feet, and a dip of the underlying strata from the sea- 

 ward, as well as from inland, has in all probability been caused. 

 This Artesian well was carried to the depth of 1524 feet, and passed 

 through three water-bearing bed;— one in the volcanic ashes, the 

 second in the sub-Apennine beds and the third in the Cretaceous 

 strata at the bottom. No eruption of lava can then take place 

 without coming in contact with these underground waters. The 

 first to be affected will be the water in the cavities of the moun- 

 tain in and around the crater. As the pressure of the a' cending 

 coluain of lava splits the crust formed subsequently to the pre- 

 ceding eruption, the water finds its way to the heated surface, 

 and leads to explosions more or less violent. When the fluid 

 lava breaks more cjmpletely through the old cru^t, and the 

 mountain is fissured by the force and pressure of the a'^cending 

 colu:nn, the whole body of water stored in the mountain succes- 

 sively flows in upon the heated lava, and is at once flushed off 

 into steam. Then take place those more violent detonations and 

 explo-ions — tho e deluges of rain arising from the condensed 

 steam — with which the great eruptions u-ually commence. In 

 conclusion, the author conceives that the first cau e of volcanic 

 action is the welling up of the lava in consequence of pressure 

 due to slight contraction of a portion of the earth's cni-t. 

 Secondly, the fluid lava coming into contact with water stored in 

 the crevices of the masses of lava and ashes forming the volcano, 

 the water is at once flushed into steam, giving rise to powerful 

 detonations and explosions. Thirdly follows an influx of water 

 from the underlying sedimentai-y or other strata lying at greater 

 depths into the ducts of the volcano ; and, lastly, as the^e sub- 

 terranean bodies of water are thui converted into steam and 

 expelled, the exhausted strata then serve as a channel to an influx 

 of sea-^vater into the volcano. A point is finally reached wdien, 

 owing to the cessation of the powerful shocks and vibrations, and 

 the excessive drainage of the sti-ata, the flow of the lava is efl^ected 



quietly, and so continues until another equilibrium is established 

 and the lava ceases to escape. 



The Connection bttween the Intrusion of Volcanic Action, by 

 Prof. W. J. Sollas, M.A. — In a volcanic eruption there are 

 concerned first the elevation of the lava column in the axial 

 pipe of the volcano, and next the explosion by which the lava 

 is ejected into the air. The author attempts to find a vera cauia 

 for the latter. Sorby's researches on included cavities prove 

 that steam at a high tension mu t have been everywhere present 

 throughout plutonic rocks when these were in a state of fusion, 

 and the presence of steam in ejected lava is well known. He 

 considers it probable that the axial pipe of a volcano is occupied 

 by fused rocks permeated by steam, which is probably in a liquid 

 state, and the tension of which will depend on the hydrostatic 

 pres^ m'e due to the lava column above it. Any sudden diminution 

 of this pres:ure will tend to a su-lden expansion of the steam, 

 and ter.d to produce a volcanic explosion. The mere elevation 

 of the lava in the volcanic pipe cannot directly produce -a 

 diminution of pressure, though an overflow at the surface of the 

 gr jund would, but this infers that the overflow of lava should pre- 

 cede an eruption, which is not the case ; hence the author con- 

 cludes that an overflow of lava from the sides of the pipe and 

 other places underground, and the pressure on the lava column 

 being reduced beneath the point of overflow, an eruption follows. 

 The ascendant pressure of intruded sheets and dykes of igneous 

 rock known to occur beneath volcanic cones thus stands in close 

 connection with the production of volcanic explosions. 



A Reitordtion of iiie Skeleton of Archinofteryx, with some 

 Remarks on Differences betzveen the Berlin and London Speci- 

 mens. — Prof. H. G. Seeley, F.R.S., traced the forms of the 

 bones from a photograph, and arranged the skeleton so as to 

 repre-ent a bird which stood about ten inches high. The head 

 has a post-occipital process in the cormorants ; the neck is curved 

 forward ; the tail reached almost to the ground ; and the limbs 

 \\ere exactly as in birds. 



On Simosauriis pitsilhn (Fraas), a Step in the Evolution of 

 the Plesiosauria. — Prof. H. G. Seeley gave a detailed de-cription 

 of the skeleton of Simo-aurus recently discovered in the Trias 

 near Stuttgart, and briefly noticed and figured by Dr. Oscar 

 Fraas He then drew special attention to the difference from 

 Ple.-iosaurus, especially in the form of the pectoral arch and in 

 the characters of the fore and hind limbs. The hind limb was 

 discussed, to show how it might a-sume like character with the 

 : ore limb. Prof. Seeley concluded that the Ple^iosaurus were 

 originally land animal , and that their ancestors and affinities 

 mast be sought in Simo>auru5, Notlrojaurus, and allied types ol 

 amphibious Triassic reptiles. 



Influence of Barometric Pressure on the Discharge of Watei 

 from Springs, by Baldwin Latham, M.Inst.C.E. — The author of 

 this paper mentioned that it was alleged, by some of the long-esta- 

 lili-hed millers on the chalk streams, that they were able to 

 foretell the appearance of rainfall from a sensible increase in the 

 volume of water flowing down the stream before the period of 

 rainfall. He had, therefore, undertaken a series of observations 

 to investigate the phenomena, and he found, in setting up gauges 

 in the Bourne flow in the Caterhara Valley, near Croydon, in the 

 spring of this year (18S1), and selecting periods when there was 

 no rain to vitiate the results, that whenever there was a rapid 

 fall in the barometer, there was a corresponding increase in the 

 volume of water flowing, and with a rise of the barometer, there 

 was a diminution in the flow. The gaugings of deep wells also 

 confirmed these oliservations ; for where there was a large 

 amount of water held by capillarity in the strata above the water- 

 line, at that period of the year when the wells became sensitive 

 and the flow from the strata was sluggi-h, that a fall in the 

 liarometer coincided with a rise in the water-line, and that under 

 conditions of high barometric pressure the water-line was 

 lowered. Percolating gauges aLo gave similar evidence, for 

 after percolation had ceased and the filter was apparently dry, a 

 rapid fall of the barometer occiu'ring, a small quantity of water 

 passed from the percolating gauges. The conclu ion arrived at 

 was, that atmospheric pressure exercises a marked influence upon 

 the e cape of water from springs. 



On Evapjration and Excentricity as Co-factors in the Causes oj 

 Glacial Epoch, by the Rev. E. Hill, M.A. 



On some Points in the Morphology of the Rhabdophora, by John 

 Uopkinso;!. — The author, afcer reviewing the characteristics o.'' 

 the group, concludes from his investigation into the morphology 

 of this group that they are the Palaeozoic representatives of the 

 recent Hydroida. 



