594 



NA TURE 



{Aprils 1885 



available supply of water becomes exhausted, or the channels of 

 communication impeded, and this continues until, with the 

 ceasing of the extravasation of the lava, the eruption comes to 

 an end. 



The author then explains the way in which the water may 

 gain access to the lava in the duct, notwithstanding heat and 

 pressure. This he considers to be dependent upon the difference 

 between the statical and the kinetical pressure of the column of 

 lava on the sides of the duct. In the change from the one state 

 to the other, when the lava begins to flow, and its lateral pres- 

 sure is lessened, the equilibrium with the surrounding elastic 

 high pressure vapour becomes destroyed, and the vapour forces 

 its way into the ascending lava. As this proceeds, the heated 

 water further from the duct, and held back by the pressure of 

 the vapour, flashes into steam to supply its place. If that water 

 should be lodged in the joints of the surrounding rock, blocks of 

 it will also be blown off, driven into, and ejected with, the 

 ascending lava, as have been the blocks in Sonvma and of other 

 volcanoes. 



It is the double action thus established between the inland- 

 and sea-waters that has probably prolonged the activity of the 

 existing volcanoes settled in ocean centres, or along coast-lines, 

 while the great inland volcanic areas of Auvergne, the Eifel, 

 Central Asia, &c. , have become dormant or extinct. 



But if water only plays a secondary part in volcanic eruptions, 

 to what is the motive power which causes the extravasation of 

 the lava to lie attributed ? This involves questions connected 

 with the solidity of the globe far more hypothetical and difficult 

 of proof. The author first takes into consideration the probable 

 thickness of the earth's crust from a geological point of view, 

 and shows, that although the present stability of the earth's 

 surface renders it evident that the hypothesis of a thin crust 

 resting on a fluid nucleus is untenable, it is equally difficult to 

 reconcile certain geological phenomena with a. globe solid 

 throughout, or even with a very thick crust. The geological 

 phenomena on which he relies in proof of a crust of small thick- 

 ness, are: — (1) Its flexibility as exhibited down to the most 

 recent mountain uplifts, and in the elevation of continental 

 areas. (2) The increase of temperature with depth. (3) The 

 volcanic phenomena of the present day, and the outwelling of 

 the vast sheets of trappean rocks during late geological periods. 



He considers that the squeezing and doubling up of the strata in 

 mountain-chains — as for example, the 200 miles of originally 

 horizontal strata in the Alps, crushed into a space of 130 miles 

 (and in some cases the compression is still greater) — can only be 

 accounted for on the assumption of a thin crust resting on a 

 yielding substratum, for the strata have bent as only a free sur- 

 face-plate could to the deformation caused by lateral pressure. 

 If the globe were solid, or the crust of great thickness, there 

 would have been crushing and fracture, but not corrugations. 

 Looking at the dimensions of these folds, it is evident also that 

 the plate could not be of any great thickness. This in connec- 

 tion with the increase of heat with depth, and the rise of the 

 molten lava through volcanic ducts, which, if too long, would 

 allow the lava to consolidate, leads the author to believe that 

 the outer solid crust may be less even than twenty miles thick. 



That the crust does possess great mobility is shown by the 

 fact that since the Glacial period there have been movements of 

 continental upheaval — to at least the extent of 1500 to 1S00 feet 

 — that within more recent times they have extended to the 

 height of 300 to 400 feet or more, and they have not yet entirely 

 ceased. 



With regard to the suggestion of the late Prof. Hopkins that 

 the lava lies in molten lakes at various depths beneath the sur- 

 face, the author finds it difficult to conceive their isolation as 

 separate and independent local igneous centres, in presence of 

 the large areas occupied by modern and by recently extinct vol- 

 canoes. But the chief objection is, that if such lakes existed 

 they would tend to depletion, and as they could not be replen- 

 ished from surrounding areas, the surface above would cave in 

 and become depressed, whereas areas of volcanic activity are 

 usually areas of elevation, and the great basaltic outwellings of 

 Colorado and Utah, instead of being accompanied by depression, 

 form tracts raised 5000 to 12,000 feet above the sea-level. 



These slow secular upheavals and depressions, this domed 

 elevation of great volcanic areas, the author thinks most com- 

 patible with the movement of a thin crust on a slowly yielding 

 viscid body or layer, also of no great thickness, and wrapping 

 round a solid nucleus. The viscid magma is thus compressed 

 between the two solids, and while yielding in places to com- 



pression, it, as a consequence of its narrow limits, expands in 

 like proportion in conterminous areas. As an example, he 

 instances the imposing slow movements of elevation which have 

 so long been going on along almost all the land bordering the 

 shores of the Polar Seas, and to the areas of depression which 

 so often further south subtend the upheaved districts. 



With respect to the primary cause of these changes and of the 

 extravasation of lava, the author sees no hypothesis which meets 

 all the conditions of the case so well as the old hypothesis of 

 secular refrigeration and contraction of a heated globe with a 

 solid crust — not as originally held, with a fluid nucleus, but with 

 the modifications which he has named, and with a quasi rigidity 

 compatible with the conclusions of the eminent physicists who 

 have investigated this part of the problem. Although the loss of 

 terrestrial heat by radiation is now exceedingly small, so also is 

 the contraction needed for the quantity of lava ejected. Cordier 

 long since calculated that, supposing five volcanic eruptions to 

 take place annually, it would require a century to shorten the 

 radius of the earth to the extent of 1 mm., or about ~} r inch. 



The author therefore concludes that, while the extravasation 

 of the lava is due to the latter cause, the presence of vapour i-; 

 due alone to the surface and underground waters with which it 

 comes into contact as it rises through the volcanic duct, the 

 violence of the eruption being in exact proportion to the quantity 

 which so gains access. 



Geologists' Association, April 9. — A short paper entitled 

 Notes on the Oldhaven pebble-beds at Caterham was read by 

 Mr, T. V. Holmes, F.G. S.. The workmen in the gravel-pits 

 adjoining the Caterham Waterworks recently exposed a large 

 cavity in the pebble-beds, which was visible when the writer 

 and Mr. R. Meldola visited the spot in December last. It was 

 cylindrical in shapie, from ten to eleven feet in length, and from 

 five to six in diameter, its axis being nearly vertical. Evidence 

 of the existence of others was noted, and it was stated that 

 similar hollows are by no means rare in these pits. These 

 cavities doubtless owed their origin to the existence of pipes in 

 the chalk beneath, which pipes, from the superior tenacity, here 

 and there, of the upper strata of gravel as compared with the 

 lower, had not been entirely filled up. Examples of similar 

 hollows elsewhere were given. The existence of masses of un- 

 modified "loam with flints " in the midst of the pebbles was 

 also noted, and the writer explained how they might be 

 accounted for without recourse to the hypothesis of glacial agency. 

 —On the Glacial Drifts of Norfolk, by Mr. H. B. Woodward, 

 F.G.S. After describing the general characters of the drifts in 

 Norfolk, Mr. Woodward alluded to the difficulties in identifying 

 the subdivisions in different areas, for the beds are variable and 

 no infallible guides are furnished by lithological characters, 

 fossils, or even by stratigraphical sequence. Looked at in a 

 broad way, two divisions might be made out: (1) the Lower 

 Glacial, including the Cromer Till, Contorted Drift, and the so- 

 called Middle Glacial ; and (2) the Upper Glacial, including the 

 chalky boulder clay and cannon-shot gravel. These divisions 

 are borne out in part by the evidence of superposition and by 

 the character of the stones imbedded in the boulder clays, and 

 in part by the evidence that the contortions in the Lower 

 Glacial beds were produced by the agent which formed the 

 chalky boulder clay. Mr. Woodward expressed his opinion that 

 the shells found in the Middle Glacial sands did not belong to 

 the Glacial period, but were derived in part from Pliocene strata 

 north of Norfolk, now either entirely removed or buried up 

 beneath the waters of the North Sea. The shells, which include 

 forms that lived during the period of the coralline and red crags, 

 were supposed by some authorities to have migrated from the 

 Mediterranean area during submergence of the tract in Glacial 

 times and at an interval when the climate was mild. Attention 

 was drawn to the occurrence of boulder clay in the Middle 

 Glacial deposits near Hertford ; and it was pointed out that 

 sheik derived from the coralline and ?red crags had been found 

 by Mr. T. F. Jamieson in the drift of Aberdeenshire, indicating 

 that Pliocene deposits had formerly extended as far north as 

 Scotland. Briefly alluding to the method of formation of the 

 glacial drifts, Mr. Woodward passed on to notice the occurrence 

 of Palaeolithic implements. The mammalian remains associated 

 with tlie^e belonged to the group which characterised the older 

 Thames Valley deposits and were met with also on the Dogger 

 Bank. When these deposits were accumulated, probably the 

 Ouse joined the waters of the Thames and Rhine in the area 

 now covered by the North Sea. 



