Sept. 6, 1883] 



NA TURE 



449 



SOME UNSOLVED PROBLEMS IN GEOLOGY^ 



1V/T Y predecessor in office remarked, in the opening of his ad- 

 dress, that two courses are open to the retiring president of 

 thi- As ociation in preparing the annual presidential discourse, — 

 he may either take up some topic relating to his own specialty, 

 or he may deal with various or general matters relating to science 

 and its progress. A geologist, however, is not necessarily tied 

 up to one or the other alternative. His subject covers the whole 

 history of the earth in time. At the beginning it allies itself with 

 astronomy and physics and celestial chemi-try. At the end 

 it runs into human history, and is mixed upw th archaeology and 

 anthropology. Throughout its whole course it has to deal with 

 questions of meteorology, geography, and biology. In short, 

 there is no department of physical or biological science with 

 which geology is not allied, or at least on which the geologist 

 may not presume to trespass. When, therefore, I announce as 

 my subject on the present occasion so ne of the unsolved pro- 

 blems of this univer.-al science, you need not be surprised if I 

 should be somewhat discursive. 



Perhaps I shall begin at the utmost limits of my subject by 

 remarking that in matters of natural and physical science we are 

 met at the outset with the scarcely solved question as to our own 

 place in the nature which we study, and the bearing of this on 

 the difficulties we ei counter. The organism of man is decidedly 

 a part of nature. We pi ice ourselves, in this aspect, in the sub- 

 kingdom vertebrata, and class mammalia, and recogni e the fact 

 that man is the terminal link in a chain of being extending 

 throughout geological time. But the organism is not all of man ; 

 and, when we regard man a, a scientific animal, we raise a new 

 question. If the human mind is a part of nature, then it is 

 subject to natural law ; and nature includes mind as well as 

 matter. On the other hand, without bein^ absolute idealist;, we 

 may hold that mind is more po'ent than matter, and nearer to 

 the real essence of things. Our science is in any case neces-arily 

 dualistic, being the product of the reaction of mind on nature, 

 and must be largely subjective and anthropomorphic. Hence, 

 no doubt, ari e much of the controversy of science, and much of 

 the unsolved difficulty. We reognise this when we divide 

 science into that which is experimental, or depends on apparatus, 

 and that which is observational an I cla sificatory, — distinctions, 

 these, which relate not a> much to the ooject; of science as to 

 our methods of pursuing them. This view al-o opens up to us 

 the thought that the domain of science is practicilly boundless ; 

 tor who can set limits to the action of mind on the universe, or 

 of the universe on mind? It follows that science mu t l>e 

 limited on all side-- by unsolved mysteries; and it will nol 

 any good purp >se to meet these with clever guesses. If we so 

 treat the en'gmasof the sphinx Nature, we shall surely be devoured. 

 Nor, on the other hand, must we collapse into absolute despair, 

 and re-ii;n ourselves to the confession of inevitable ignorance. 

 It becomes us, rather, boldly to confront the unsolved questions 

 of nature, and to wrestle with their difficulties till we master such 

 as wc can, and cheerfully leave those we cannot overcome to be 

 grappled with by our successors. 



Fortunately, as a geologis', I do not need to invite your 

 attention to those transcendental questions which relate to the 

 ultimate constitution of matter, the nature of the ethereal medium 

 filling space, the absolute difference or identity of chemical 

 elements, the cause of gravitation, the conservation and dissipa- 

 tion of energy, the nature of life, or the primarv origin of 

 biopla-mic matter. I may take the much more humble role 

 of an inquirer into the unsolved or partially solved problems 

 which meet us in considering that short and imperfect record 

 which geology studies in the rocky layers of the earth's crust, and 

 which leads no farther back than to the time when a solid rind 

 had already formed on the earth and was already covered with 

 an ocean. This record of geology covers but a small part of the 

 history of the earth and of the system to which it belongs, nor 

 does it enter at all into the more recondite problems involved ; 

 ■ till it forms, I believe, some necessary preparation, at least, to 

 the comprehension of these. 



What do we know of the oldest and most primitive rocks ? 

 At this moment the question may be answered in many and di - 

 cordant ways ; yet the leading elements of the answer nay 

 be given very simply. The oldest rock formation known to 



1 Address of the retiring president of the American Association fsr the 

 Advancement of Science, ^Principal J. W. Dawson. 1X.D., F.R.S., at 

 Minneapolis, August 15, 1883. Advance proofs of this and other 

 addresses to follow have been kindly sent us by the Editor of Science. 



geologists is the lower Laurentian, the fundamental gneiss, the 

 Lewisian formation of Scotland, the Ottawa gneiss of Canada. 

 This formation of enormous thickness corresponds to what the 

 older geologists called the fundamental granite, — a name not 

 to be scouted, for gneiss is only a stratified granite. Perhaps 

 the main fact in relation to this old rock is that it is a gneiss ; 

 that is, a rock at once bedded and cry talline, and bavin; for its 

 dominant ingredient the nvneral orthoclase, — a compound of 

 silica, alumina, and potash, — in which are embedded, as in a 

 paste, grains and crystals of quartz and hornblende. We know 

 very well, from its texture and composition, that it cannot be a 

 product of mere heat ; and, being a bedded rock, we infer that 

 it was laid down layer by layer, in the manner of aqueous 

 deposits. On the other hand, its chemical composition is quite 

 d fferent from that of the muds, sands, and gravels usually 

 deposited from water. Their special characters are caused by 

 the fact that they have resulted from the slow decay of rocks 

 like thee gneisses, under the operation of carbonic acid and 

 water, whereby the alkaline matter and the more soluble part 

 of the silica have been washed away, leaving a residue mainly 

 siliceous and aluminous. Such more modern rocks tell of dry 

 land subjected to atmospheric decay and rainwa h. If they 

 have any direct relation to the old gneisses, they are their grand- 

 children, not their parents. On the contrary, the olde t gneisses 

 show no pebble-, or sand, or limestone — nothing to indicate that 

 there was then any land undergoing atmospheric waste, or shores 

 with sand and gravel. For all that we know to the contrary, 

 these old gneisses may have been deposited in a shoreless sea, 

 holding in solution or suspen-ion merely what it could derive 

 from a submerged crust recently cooled from a stale of fusion, 

 still thin, and exuding here and there through its fissures heated 

 waters and volcanic products. 



It is scarcely necessary to say thai I h.ove no confidence in the 

 supposition of unlike composition f the earth's mass on different 

 sides, on which Dana has partly based his theory of the origin 

 of c mtinents. The most probable conceptio i seems to be that 

 of Lyell ; namely, a molten mass, uniform except in so far a- 

 denser material might exist towards its centre, and a crust, at 

 first approximately even and homogeneous, and subsequently 

 thrown into great bendings upward and downward. This 

 question has recently been ably discussed by Mr. Crosby in the 

 London Geological Magazine.' 1 



In short, the fundamental gneiss of the lower Laurentian may 

 have been the first rock ever formed ; and in any case it is a rock 

 firmed under conditions which have not -ince recurred, except 

 locally. It constitutes the first and best example of these 

 chemic ■>- physical, aqueous, or aqueo-igneous rocks, so character- 

 i- tic of the earliest period of the earth's history. Viewed in 

 this way, the lower Laurentian gneiss is probably the oldest kind 

 of rock we shall ever know, — the limit to our backward pro- 

 gress, beyond which there remains nothing to the geologist 

 except physical hypotheses respecting a cooling, incandescent 

 globe. For the chemical conditions of these primitive rocks, 

 and what is known as to their probable origin, I must refer you 

 to my friend Dr. Sterry Hunt, to whom we owe so much of 

 what is known of the older crystalline rocks, 2 as well as of their 

 literature and the questions which they raise. My purpose here 

 is to sketch the remarkable difference which we meet as we 

 ascend into the middle and upper Laurentian. 



In the next succeeding formation, the true lower Laurentian 

 of Logan, the Grenville series of Canada, we meet with a great 

 and significant change. It is true, we have still a predominance 

 of gneisses which may have been formed in the same manner 

 with those below them ; but we find the-e now associated with 

 great beds of limestone and dolomite, which must have been 

 formed by the separation of calcium and magnesium carbonates 

 from the sea water, either by chemical precipitation or by the 

 agency of living beings. We have also quartzite, quartzose 

 gneisses, and even pebble beds, which inform us of sand-banks 

 and shores. Nay, more, we have beds containing graphite, which 

 must be the residue of rolants, and iron ores which tell of the 

 deoxidation of iron oxide by organic matters. In short, here 

 we have evidence of new factors in world-building, — of land and 

 ocean, of atmospheric decay of rocks, of deoxidising processes 

 carried on by vegetable life on the land and in the waters, of 

 lime-tone-building in the sea. To afford material for such rocks, 

 the old Ottawa gneiss must have been lifted up into continents 

 and mountain masses. Under the slow but sure action of the 

 carbonic dioxide dissolved in rain water, i s felspar had crumbled 

 1 June, 1883. a Hunt. '■ Essays on Chemical Geology." 



