Nov. 2 2, 1888] 



NATURE 



93 



rent the possibility of misstatement I will give it as a quota- 

 ' To a geologist (especially one belonging to the school 

 Lyell) it is equally difficult to conceive that there should be' a 

 " distinction between the metamorphic rocks of Archaean 

 pjst-Archaean age respectively, as that the pre-Tertiary 

 Icanic rocks should be altogether different in character from 

 of Tertiary and recent times." Of course in this state- 

 it much depends on the sense attached to the epithet "broad." 

 an abstract proposition I should admit, as a matter of course, 

 It from similar causes similar consequences would always 

 low. But in the latter part of the quotation lurks a petilio 

 incipil. During the periods mentioned volcanic rocks appear, 

 we should expect, to have been ejected from beneath the 

 th's crust similar in composition and condition, and to have 

 idified with identical environment. Hence the results, allow- 

 for secondary changes, should still be similar. But to 

 jme that the environment of a rock in early Archjean times 

 identical with that of similar material at a much later 

 mod is to beg the whole question. My creed, also, is the 

 uniformitarian ; but this does not bind me to follow a formula 

 to a position which is untenable. Other studies with which I 

 re some familiarity have warned me that a blind orthodoxy is 

 of the best guides to heresy. "The weakness and the 

 ical defect of uniformitarianism " — these are Prof. Huxley's 

 s — "is a refusal, or at least a reluctance, to look beyond 

 present order of things. ' and the being content for all 

 ae to regard the oldest fossiliferous rocks as the Ultima 1 hide 

 \ our science." Now, speaking for myself, I see no evidence 

 ce the time of these rock-, as at present known, of any very 

 terial difference in the condition of things on the earth's sur- 

 The relations of sea and land, the climate of regions, 

 re been altered ; but because 1 decline to revel in extem- 

 ized catastrophes, and because I believe that in Nature order 

 prevailed and law has ruled, am I therefore to stop my 

 juiries where life is no longer found, and we seem approaching 

 firstfruits of the creative power? Because palaeontology is, 

 force, silent; because the geologist can only say, "I know 

 more," must I close my ear to those who would turn the 

 It of other sciences upon the dark places of our own, and 

 ;t their reasoning with the exclamation, " This is not written 

 L the book of uniformity"? To do this would be to imitate 

 silversmiths of old, and silence the teacher by the cry, 

 jreat is Diana of the Ephesians." 

 ;What, then, does the physicist tell us was the initial condition 

 ^this globe ? I will not go into the vexed question of geological 

 le, though as a geologist I must say that we have reason to 

 iplain of Sir W. Thomson. Years ago he reduced our credit 

 Ithebank of time to a hundred millions of years. We grumbled, 

 t submitted, and endeavoured to diminish our drafts. Now 

 has suddenly put up the shutters, and declared a dividend of 

 than four shillings in the pound. I trust some aggrieved 

 ireholder will prosecute the manager. However, as a cause 

 bre is too long a business for the end of an evening, I will 

 trely say that, while personally I see little hope of arriving at 

 "ironological scale for the age of this earth, I do not believe 

 its eternity. What, then, does the physicist tell us must 

 Ive been in the beginning? I pass by those earliest ages, 

 s "Ilion, like a mist, rose into towers," so from the 

 swing cloud the great globe was formed. I pass on to a con- 

 ion more readily apprehended by our faculties — the time, the 

 sisteutior status of Leibnitz, when the molten globe had 

 sted over, and its present history began. Rigid uniformi- 

 m though you may be, you cannot deny that when the very 

 face of the ground was at a temperature of at least 1000" ¥., 

 there was no rain, save of glowing ashes — no river, save of molten 

 fire. Now is ending a long history with which the uniformitarian 

 must not reckon — of a time when many compounds now existing 

 were not dissolved but dissociated, for combination under that 

 environment was impossible. Yet there was still law and still 

 order — nay, the present law and order maybe said even then 

 to have had a jiotential existence — nevertheless to the uniformi- 

 tarian gnome, had such there been, every new combination of 

 elements would have been a new shock to his faith, a new 

 miracle in the earth's history. But at the times mentioned 

 above, though oxygen and hydrogen could coinbine, water 

 could not yet rest upon the ruddy crust of the globe. 

 What does that mean? This, that assuming the water of the 

 ocean equivalent to a spherical shell of the earth's radius 

 and two miles thick, the very lava-stream would consolidate 

 under a pressure of about 310 atmospheres, equivalent to nearly 



4C00 feet of average rock.^ But on the practical bearing of this 

 consideration I will not dwell. Let us pass on to a time which, 

 according to Sir W. 1 homson, would rather quickly arrive, 

 vhcn the surface of the crust had cooled by radiation to its 

 present temperature. Let us, merely for illu:tration, take a 

 surface temperature of 50° F. (nearly that of London), and 

 assume that the present rise of crust temperature is 1° F. for 

 every 50 feet of descent, which is rather too rapid. If so, 

 212° F. is reached at 8ico feet, and 250" F. at 10,000 feet. 

 Though the latter temperature is far from high, yet we should 

 expect that under such a pressure chemical changes would occur 

 with much more facility than at the surface. But many Palaeo- 

 zoic or even later rock masses can now be examined which at a 

 foimer period of their history have been buried beneath at least 

 10,000 feet of sediment ; yet the alteration of their constituents 

 has been small : only the more unstable minerals have been 

 somewhat modified, the more refractory are unaffected. But 

 for a limited period after the consiitentior status, the increase of 

 crust temperature in descending would be far more rapid ; when 

 one-twenty-fifth of the whole period from that epoch to the pre- 

 sent had elapsed, and this is no inconsiderable fraction, the rate 

 of increase would be l" for every 10 feet of descent. Suppose, for 

 the sake of comparison, the surface temperature as before, the 

 boiling-point of water would be reached at 1620 feet, and at 

 10,000 feet, instead of a temperature of 250° F., we should have 

 one of 1050° F. But at the latter temperature many rock 

 masses would not be perfectly solid.-' According to Sorby, the 

 steam cavities in the Ponza tiachyte must have formed, and thus 

 the rock have been still plastic at so low a temperature as 

 680° F. At this period, then, the end of the fourth year of the 

 geological century, whatever I e its units, structural changes in 

 igneous and chemical changes in sedimentary rocks must have 

 occurred more readily than in any much later period in the 

 world's history. A temperature of 2000° F., sufficient to melt 

 silver — more than sufficient to melt many lavas — would have 

 been reached at a depth of about 4 miles. It would now be 

 necessary to descend for at least 20 miles in order to arrive at 

 this zone. It, during the ninety-six years of the century, has 

 been changing its position in the earth's crust, more slowly as 

 time went on, from the one level to the other. 



There is another consideration, too complicated for full 

 discussion, too uncertain, perhaps, in its numerical results 

 to be more than mentioned at present, which, however, 

 seems to me important. It is this, that in very early times, 

 as shown by Prof Darwin and Mr. Davison, the zone in 

 the earth's crust, at which lateral thrust ceases and tension 

 begins, must have been situated much nearer to the surface 

 than at present. If now, at the end of the century, it is at the 

 depth of 5 miles, it was, at the end of the fourth year, 

 at a depth of only i mile. Then, a mass of rock, 10,000 

 feet below the surface, would be nearly a mile deep in the zone 

 of tension. Possibly this may explain the mineial banding of 

 much of our older granitoid rock, already mentioned, and the 

 coincidence of foliation with what appears to be stratification in 

 the later Archasan schists, as well as the certainly common coin- 

 cidence of microfoliation with bedding in the oldest indubitable 

 sediments. 



Pressure, no doubt, has always been a most important factor 

 in the metamorphism of rocks ; but there is, I think, at present 

 some danger in over-estimating this, and representing a partial 

 statement of truth as the whole truth. Geology, like many 

 human beings, suffered from convulsions in its infancy ; now, in 

 its later years, I apprehend an attack of pressure on the brain. 



The first deposits on the solidified crust of the earth would 

 obviously be igneous. As water condensed, denudation would 

 begin, and stratified deposits, mechanical and chemical, become 

 possible, in addition to detrital volcanic material. But at that 

 time the crust itself, and even stratified deposits, would often be 

 kept for a considerable period at a temperature similar to that 

 afterwards produced by the invasion of an intrusive mass. Thus 

 not only rocks of igneous origin (including volcanic ashes) would 

 predominate in the lowest foundation-stones, but also secondary 

 changes would occur more readily, and even the sediments or 

 precipitates should be greatly metamorphosed. Strains set up 

 by a falling temperature would produce, in masses still plastic, 

 banded structures, which, under the peculiar circumstances, 



' If we tal-e the specific gravity of water as unity, and that of mean rock 

 as 2'7, ihe Fte^sure would be = 39ifi feet of rock. 



" 1 he lowtst temperature, which, so far as I knew, has been observed in , 

 lava tbasic) while still plastic, is 1228° F. 



