Dec. 26, 1889] 



NATURE 



175 



not "partners," for the latter does not participate in the gain of 

 the former. How, then, on symbiotic principles, can "natural 

 selection " have been the means of producing a growth which, 

 though important, if not necessary, to the animal symbiont, is 

 more or less prejudicial to the symbiont vegetable organism? 



There can, of course, be no doubt, as Mr. McLachlan says, 

 that the various peculiarities of gall-structure " could be" ex- 

 plained " on purely physiological grounds if carefully studied ; " 

 but that "natural selection" will suffice to explain them, seems 

 to me by no means equally free from uncertainty. 



St. George Mivart. 



Hurstcote, Chilworth, December 13. 



The Permanence of Continents and Oceans. 



I CAN find no flaw in the reasoning on the dynamical ques- 

 tion of the permanence of continents and oceans, in Mr. Starkie 

 Gardner's letter in Nature of December 5 (p. 103), by which 

 he endeavours to show the universal "tendency for deep oceans 

 to become deeper, and for mountain chains to grow into higher 

 peaks." But when he says it is opposed to no known facts, I 

 wish to ask how it is to be reconciled with the fact of the 

 general distribution of marine deposits over the face of the 

 earth, so that every part of what is now land appears to have 

 once been ocean ? 



I fully concede that the change of ocean spaces into land 

 spaces is an extremely slow process, taking, probably, millions 

 of years, but it seems to me that it must have occurred, though 

 I cannot suggest through what agency. 



Belfast, December 14. Joseph John Murphy. 



Does the Bulk of Ocean Water Increase? 



Mr. Jukes-Browne (Nature, December 12, p. 130) admits 

 that " if the area of the land were larger, and the depth of the 

 oceans less," in early geological times, a further inference must be , 

 drawn — "that the bulk of the ocean water was less then than it is ' 

 now." ^ 



So far we are in agreement ; indeed, we could scarcely be " 

 otherwise, as the proposition admits of complete demonstration. 

 When, however, Mr. Jukes-Browne proceeds to give his reasons ', 

 for holding that the bulk of ocean water was less in early times 

 than now, he enters upon a more controversial subject. ; 



1 am familiar with the arguments he urges partly on the 

 authority of Mr. Fisher, and have to some extent discussed them 

 in chapter xii. of the "Origin of Mountain Ranges." I desire, 

 however, to point out a further objection that when stated will, 

 I think, appear extremely obvious. 



According to Dr. George Darwin and many other astronomers 

 who follow him, our satellite, the moon, was once an integral 

 portion of the earth, having been thrown off when the earth was 

 in a molten condition. If this theory be correct, it is a fair as- 

 sumption that the magma out of which the moon has consolidated 

 was composed of matter similar to that of our earth. Even if 

 their relations were never so intimate as this, I think most 

 physicists and astronomers will admit a similarity of material 

 constitution of the two spheres. 



If then volcanic action on the earth is, as Mr. Jukes-Browne 

 contends, accompanied by a separation of water initially con- 

 tained in the magma, and its condensation on the surface in such 

 quantities as to materially increase the bulk of ocean water, why 

 has not the same effect followed volcanic action on the moon ? 

 Why, in fact, do we not see oceans on the surface of the moon 

 instead of a dry and desert waste of volcanic rings, mountain 

 protuberances, and arid plains ? In face of this great fact it ap- 

 pears to me that ingenious arguments as to the amount of water 

 contained in the fluidal cavities of granite, which most geologists 

 think is explicable by percolation, have not much weight. 



At all events, it seems a reasonable question to ask why 

 oceans should be supplied with water from the perspiring pores 

 of mother earth, while her offspring, the moon, is so dry as to 

 have absorbed into herself all evidence of any aqueous envelope 

 that may have formerly existed. T. Mellard Reade. 



Park Comer, Blundellsands, December 14. 



A Natural Evidence of High Thermal Conductivity in 

 Flints. 

 A RATHER curious effect of the recent frost attracted my 

 attention in the gravel foot-paths leading over Addington Hill, 



near Croydon, on the beautifully bright day of the ist inst. 

 The clear nights and frosty air of the closing week of last month 

 had been productive of continued low temperatures in that 

 locality, and the result observed was that the flint pebbles, 

 which in neighbouring gravel-beds and here and there on the 

 paths, are of the size of hens' eggs, and remarkably well rounded, 

 had, in places, sunk in the frozen clunch or clay-earth of the 

 foot-paths, and in the peaty ground or turf beside the paths, as 

 it appeared, like filberts shrunk and resting at the bottoms of 

 their shells ; or else as if the pebbles' earthy moulds had, by 

 expanding upwards, left such a large vacuity above each stone, 

 that the tops of some of the large ones, instead of being level 

 (as at first they must have been, by the appearance of the moulds) 

 with the surface of the ground, were now, in a somewhat turfy 

 place, about as much as half an inch below it. The physical 

 enigma which hereupon offered itself for elucidation was, how 

 the pebbles could remain at the much lower level, while such a 

 considerable expansion upwards had been brought about by 

 freezing in the moist earth immediately surrounding them ; and 

 this problem had certainly, in looking at the thickly-clustered 

 cavities in the frozen ground, at first a very paradoxical appear- 

 ance. 



But if the question how the inclosing cavities of moist earth 

 round flint pebbles which are nearly embedded in it, are dis- 

 tended upwards so curiously by a strong frost's predominance, 

 has presented, it may be, to some of your readers who may have 

 noticed in similar conditions a similar appearance, as it at first 

 did to me, a subject for rather puzzled contemplation and con- 

 jectures, it will be worth pointing out, perhaps, that there is a 

 well-ascertained thermal property of siliceous rocks and flint, of 

 which it seems not improbable that this not unfrequently occur- 

 ring action of a strong frost, in such conditions, may really be 

 an interesting illustration. 



Among a series of about a hundred different descriptions and 

 varieties of commonly occurring rocks whose thermal conducti- 

 vities were experimentally determined by a Committee of the 

 British Association in the years 1874-78, it was found that such 

 entirely siliceous ones as quartz, flint, and pure siliceous sand- 

 stone, &c., so much surpass all other ordinary rocks in their 

 rates of transmitting both heat and temperature, that in flint 

 pebbles these conducting powers are, for example, about four or 

 five times as great as in damp sandy mould, or in wet clayey 

 earth. 



Instead of the layers of cold temperature, therefore, produced 

 in wet pebbly ground by continued frosty winds and radiation, 

 proceeding in plane levels downwards from one depth below the 

 surface to another, large flints exposed in it must grow cold very 

 quickly through their whole substance, and must freeze the wet 

 earth under them almost as soon as the soil's surface-layer round 

 them is beginning to be frozen. The effect of this freezing process's 

 expansion, it seems evident, will hardly be so much to raise the 

 pebbles and the earth's exposed surface upwards very differently 

 from each other, by the frost's nearly equal action on them 

 both, as, during the frost's continuance, to force up towards the 

 surface a large superfluity of soft earth from between the bedded 

 stones, carrying the cast or mould of the stone's upper sides, 

 itself to some height above them. We would require, perhaps, 

 as an aid to this interpretation of the process, to regard the con- 

 gelation round the stones, as rooting them down, perhaps to 

 lower-lying ones, so that the upward thrust of the extruded 

 earth may not be able to dislodge them, but can be effective to 

 raise up their frozen caps ; but some such supposition as this does 

 not appear to be a very impossible conjecture. By this recourse 

 to the pre-eminent thermal conductivity of flints above that of 

 moist turf and clay, in which they are embedded, it seems at 

 least not impracticable to give a somewhat intelligible explana- 

 tion of the frozen ground's abnormal elevation round them, 

 lifting the moulded caps of earth-covering off their upper sides 

 until their roadside clusters present the curious appearance of 

 shrunken petrifactions of some nest of fossil yolks in half- 

 empty egg-shells. 



It is, indeed, true that when by long continuance of a frost 

 the sodden earth may have become entirely penetrated and 

 frozen by it to some considerable and tolerably even depth (we 

 may suppose) below a layer of embedded flints, it should be 

 noticed, to simplify the process's consideration, that the form 

 which the frozen ground will then have acquired between and 

 round the flints could be nowise affected in the end by any various 

 shapes, plane or contorted by irregularly formed and differently 

 conducting solid bodies in its course, wherewith the tract of 



