326 



NATURE 



{August 2, 1 83 3 



at the surface, how can the conclusion be avoided that 

 there is everywhere a point in the earth's crust at which 

 it must be just sufficient to keep rocks at the melting 

 point. It seems utterly impossible, if it is once conceded 

 that pressure does render rocks fluid, to avoid the con- 

 clusion that there everywhere exists a viscous substratum 

 following to some extent the contour of the earth's 

 surface. 



Such a condition is precisely that which will alone 

 explain the undoubted fact that the addition or removal 

 of even comparatively small weights produces correspond- 

 ing changes in the previous level of the earth's surface. 

 "The deposition of iooo feet of rock will, of course, cause 

 a corresponding rise in the isogeotherms," * that is, of the 

 liquid layer, and " denudation of the land must lead to a 

 depression of the isogeotherms, and a consequent cooling 

 of the upper layers of the crust." Dr. Fisher 2 proves 

 mathematically, in fact, that a liquid layer and no other 

 condition can explain the movements that have taken 

 place in the earth's crust, and satisfactorily account for 

 volcanic action. 



It appears in the present state of knowledge almost 

 impossible to estimate the depth at which a viscous layer 

 could exist. The estimates that have been made vary 

 from iooo miles to only 50,000 feet. On the one hand, 

 however, if it is a fact, as Dr. Geikie surmises, that sedi- 

 mentary formations of Silurian age have been fused and 

 rendered viscous mainly by the mere superincumbent 

 pressure of more recent sedimentary formations, the 

 depth at which a viscous layer can exist must be less 

 than the lowest estimate yet formed. On the other, the 

 observed increase of temperature, not exceeding at most 

 i° F. for every 50 feet of depth, the melting temperature 

 of rock, 2000° to 3000 F., would not be reached at a less 

 depth than 100,000 feet. This is obviously too great a 

 depth to account for some of the observed facts of 

 geology, and is without any allowance for the increasing 

 density of rocks at great depths, or for the many unknown 

 agencies which may contribute at such depths to lower 

 their melting temperature. The inert weight of 25,000 

 feet of rock of the density of slate, the thickness which ac- 

 cording to Dr. Geikie has reduced rock to a viscous state, 

 is about 2000 tons to the foot. I am not aware that 

 any estimate has been made of the actual amount of 

 heat that would be produced under such conditions. 

 If, as must be the case, any relatively small increase of 

 pressure produces a displacement in the molten layer, a 

 compensating elevation must take place elsewhere, and if 

 its effects are so considerable when the weights are rela- 

 tively small, the results of pressure applied to oceanic 

 basins must be infinitely great. The theory that oceanic 

 basins have been permanent has been embraced by many 

 of the ablest geologists, and since sediment has been 

 forming in them uninterruptedly, at however slow a rate, 

 since Eozoic times, its aggregate vertical thickness by 

 now must be colossal. The pressure of water alone upon 

 the rocks forming the bed of the greatest depths of the 

 ocean (say 4000 fathoms) would equal 6195^ tons upon a 

 square yard, and this pressure exists on a bottom which 

 is at or near the freezing-point. The effect would be as if 

 on land the pressure of the first 7000 or Sooo feet of rock 

 generated no heat whatever, or rather as if the heat were 

 intercepted by an icy layer, which also might conduct it 

 away, with the result that the molten layer would rest 

 under a greater weight under the ocean, where the rocks 

 have been observed to be denser, than it does under the 

 land. This extra weight, even if small, would tend to 

 render the greatest depths of the ocean permanent, but 

 lines of current, where sedimentation was less rapid, 

 would present lines of relatively less resistance, which, 

 becoming more and more elevated, would in time form 

 submarine ridges or banks or dry land, until the extreme 



1 Geikie, I.e., p. 287, i a F. for every 50 ftet. 



2 "Physics of the Karth's Crust. " 



tension might possibly become relieved by the eruption 

 of volcanic matter. The lines of absolutely least resist- 

 ance would, however, most frequently perhaps coincide 

 with sea-margins, because these would often be the 

 nearest lines free from the pressure of accumulating sedi- 

 ment. While, therefore, actual shore-lines may be de- 

 pressed by local sedimentation, there may be inland a far 

 more important tendency to elevation. The recent 

 mountain chains, whether volcanic or otherwise, follow at 

 a distance the contours of coasts, and it is likely that such 

 apparent exceptions as the Alps, Urals, and Himalayas 

 were in proximity to coast-lines at the time of their 

 formation. 



To this extent, I believe, the permanence of ocean 

 basins can be maintained, but the past and present dis- 

 tribution of both plants and pulmonate mollusca, which 

 alone in terrestrial life seem to have any antiquity as 

 species, appear to be wholly against any further extension 

 of it. 



That such considerations, theoretical as they seem, 

 may have a practical value to geologists, a recent journey 

 to Iceland abundantly proved. There is not only there, 

 but in the Faroes, evidence of a period of quiescence be- 

 tween two great basaltic formations, during which plants 

 grew and lignite was formed. It even appears that this 

 quiescent period extended synchronously from Ireland to 

 Greenland. During this time the Lower Eocene flora, 

 splendidly represented at Reading, seems to have migrated 

 through increasing temperature as far north as Green- 

 land, for the Reading plants are almost wholly those 

 which were thought to be characteristic of northern 

 Tertiary floras and distinctive of Miocene time. A con- 

 nection between Europe and America in these high lati- 

 tudes has also been inferred on many grounds to have 

 existed at about the same period. Does it not seem as if 

 the elevation of land (which permitted these floras thus 

 to migrate, and which probably raised the Eocene tem- 

 perature by excluding the Arctic Ocean from the Atlantic) 

 caused these stupendous eruptions of basalt to cease — for 

 elevation on such a scale must mean relief from tension — 

 and that its submergence during the Miocene led to, or 

 was caused by, the renewal of the basaltic flow ? The 

 horizon in Iceland is marked by only very scattered sedi- 

 mentary tuffs and lignites, and is far less marked than in 

 the Faroes, but in the region of Akreyri it can be traced 

 even from a distance by the highly-laminated beds of 

 light-coloured trachyte, which seem to have ushered in 

 the new volcanic activity. It would be impossible on 

 internal evidence to assign most of them to any definite 

 age, and it is only perhaps on broad considerations such 

 as these that their geological position may hereafter be 

 fixed, though their immense antiquity may be inferred 

 by the denudation which has furrowed, since their de- 

 position, nearly the whole surface of the island into deep 

 troughs and high ridges, out of what were formerly con- 

 tinuous tabular sheets of basalt. 



It would be interesting to ascertain whether the great 

 basaltic outpours of Oregon and the Deccan preceded or 

 accompanied any marked changes of level in adjoining 

 areas. 



The question has been asked as an objection to this 

 theory, What possible mechanical properties can we 

 attribute to the upper strata of the earth which will per- 

 mit them to sustain the whole of the Himalayan plateau 

 and North and South America above the sea-level, and 

 yet will cause a continuous subsidence in an estuary in 

 which sediment is being deposited ? Such subsidence, 

 it is maintained, could only occur with a substratum 

 somewhere of viscous matter, and if such viscous matter 

 exists, why does it not flow under the stresses due to the 

 weight of continents and mountains ? 



It is difficult to meet this objection except by appealing 

 to the facts. It is apparent that continents, and espe- 

 cially mountain masses, have been upheaved from below 



