5*8 



SCIENTIFIC NEWS. 



[Nov. 16, IE 



in the world's history. A temperature of 2,000° F., 

 sufficient to melt silver — more than sufficient to melt 

 many lavas — would have been reached at a depth of 

 about four miles. It would now be necessary to descend 

 for at least twenty 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 Professor 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 five miles, it was, at 

 the end of the fourth year, at a depth of only one 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 mineral banding of much 

 of our older granitoid rock, already mentioned, and the 

 coincidence of foliation with what appears to be stratifi- 

 cation in the later Archasan schists, as well as the certainly 

 common coincidence 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, mechani- 

 cal 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 con- 

 siderable pericd at a temperature similar to that after- 

 wards 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, 

 might occur in rock? now coarsely crystalline. As time 

 went on, true sediments would predominate over ex- 

 travasated materials, and these would be less and less 

 affected by chemical changes, and would more and more 

 retain their original character. Thus we should expect 

 that as we retraced the earth's course through " the 

 corridor of time," we should arrive at rocks which, though 

 crystalline in structure, were evidently in great part 

 sedimentary in origin, and should beyond them find 

 rocks of more coarsely crystalline texture and more 

 dubious character, which, however, probably were in 

 part of a like origin ; and should at last reach coarsely 

 crystalline rocks, in which, while occasional sediments 

 would be possible, the majority were originally igneous, 

 though modified at a very early period of their history. 

 This corresponds with what we find in nature, when we 

 apply, cautiously and tentatively, the principles of inter- 

 pretation which guide us in stratigraphical geology. 

 I have stated as briefly as possible what I believe to 



be facts. I have endeavoured to treat these in 

 accordance with the principles of inductive reasoning. 

 I have deliberately abstained from invoking the aid of 

 " deluges of water, floods of fire, boiling oceans, caustic 

 rains, or acid-laden atmospheres," not because I hold it 

 impossible that these can have occurred, but because I 

 think this epoch in the earth's history so remote and so 

 unlike those which followed, that it is wiser to pass it by 

 for the present. But unless we deny that any rocks 

 formed anterior to or coeval with the first beginning of 

 life on the globe can be preserved to the present time, 

 or, at least, be capable of identification — an assumption 

 which seems to me gratuitous and unphilosophical — then 

 I do not see how we can avoid the conclusion to which 

 we are led by a study of the foundation-stones of the 

 earth's crust, namely, that these were formed under 

 conditions and modified by environments which, during 

 later geological epochs, must have been of very excep- 

 tional occurrence. If, then, this conclusion accords with 

 the results at which students of chemistry and students 

 of physics have independently arrived, I do not think 

 that we are justified in refusing to accept them, because 

 they lack the attractive brilliancy of this or that 

 hypothesis, or do not accord with the words, in which a 

 principle, sound in its essence, has been formulated. It 

 is true in science, as in a yet more sacred thing, that 

 " the letter killeth, the spirit giveth life." 



CORRESPONDENCE. 



The Editor does not hold himself responsible for opinions expressed 

 by his correspondents, nor can he take notice of anonymous com- 

 munications. All letters must be accompanied by the name and 

 address of the •writer, not necessarily for publication, but as a 

 guarantee of good faith. 



THE MARTIAN CANALS. 



Your recent articles on this subject summarise the observa- 

 tional results in a thorough manner ; but the final and theo- 

 retical chapter much disappoints me. It may, possibly, have 

 been written a month or two back ; if so, the entire omission 

 is explained of what is, in my opinion, the best hypothesis 

 yet advanced— one due to Dr. Eugene Penard, of Geneva. 



This first appeared, I believe, in V Astrcmomie for October, 

 and supposes the canals to be due to inundation of fissures, 

 in the probably thick crust of the planet, produced by its 

 secular contraction. We observe such clefts on a reduced 

 scale in the moon, as well as in the Western States, the 

 Deccan, and the South of France. In the case of the larger 

 canals, we have probably a sinking en masse of the slice 

 between two parallel fissures, such as occurs terrestrially in 

 the Rhine Valley. Fractures here, as Daubree found with 

 glass, etc., would only stop at the edges, or on meeting with 

 other cracks. 



To extend this : — In the case of doubles, it is evident that 

 a general sinking between the two clefts would occur but to 

 a lesser extent, and therefore, after flooding, paler in tint. 

 This explains why in August last the Lick telescope showed 

 no duplicate canals, but simply shady bands of the width of 

 the mapped twins. 



Why, too, does your contributor dismiss the late Mr. 

 Proctor's diffraction theory with the single argument that 

 rivers do not cross continents from sea to sea? This is not 

 even a terrestrial fact ; for the Rhine and the Rhone form an 

 almost continuous waterway across Europe, the Vosges and 

 Jura Mountains preventing a straight course. Other examples 

 might be given. The real and fatal objection is, that eye- 

 pieces of different powers have not been found to give dis- 

 tances varying out of proportion to them, as they would in 

 the case of diffraction images. 



In V Astronomic for November, the indefatigable editor (M. 

 Camille Flammarion), who furnishes fully half the "copy," 



