Oct. 26, 1888.] 



SCIENTIFIC NEWS. 



441 



The secret of the earth's hot youth has doubtless been 

 well kept. So well that we have often been tempted to 

 guess idly rather to labour patiently. Nevertheless we 

 are beginning, as I believe, to feel firm ground after long 

 walking through a region of quicksands ; we are laying 

 hold of principles of interpretation, the relative value of 

 which we cannot in all cases as yet fully apprehend — 

 principles which occasionally even appear to be in con- 

 flict, but which will some day lead us to the truth. 



I shall not attempt to give you an historical summary, 

 but only to lay before you certain facts for which I can 

 answer, and to indicate the inductions which these, as it 

 seems to me, warrant. If I say little of the work of 

 others, it is not from a desire of taking credit to myself, 

 but because it is immaterial for my present purpose, who 

 first made a particular observation and how far his in- 

 ductions therefrom were correct. The acknowledgment 

 of good work would involve repudiation of bad, and for 

 that, so far as persons are concerned, it seems hardly 

 fair to use the present occasion. So, in the outset of 

 this lecture, I will once for all make a statement which I 

 have sometimes thought of invariably using, like a pre- 

 fatory invocation, " You are free to suppose that every- 

 thing herein has been said by somebody, somewhere," 

 but I will add that, as far as possible, every assertion 

 has been personally verified. 



The name Cambrian has been given to the oldest 

 in which fossils have been found. This group forms the 

 first chapter in the first volume, called Palaeozoic, of the 

 history of living creatures. Any older rocks are pro- 

 visionally termed Archaean. These — I speak at present 

 of those indubitably underlying the Cambrian — exhibit 

 marked differences one from another. Some are certainly 

 the detritus of other, and often of older, materials — 

 slates and grits, volcanic dust and ashes, even lava-flows. 

 Such rocks differ but little from the basement beds of 

 the Cambrian ; probably they are not much older, com- 

 paratively speaking. But in some places we find, in a like 

 position, rocks, as to the origin of which it is more 

 difficult to decide. Often in their general aspect they 

 resemble sedimentary deposits, but they seldom retain 

 any distinct indications of their original fragmental con- 

 stituents. They have been metamorphosed, the old 

 structures have been obliterated, new minerals have 

 been developed, and these exhibit that peculiar orienta- 

 tion, that rudely parallel arrangement, which is called 

 foliation. Except for this some masses are fairly homo- 

 geneous, while some exhibit a distinct mineral banding 

 which is usually parallel with the other structure. These 

 rocks are the gneisses and schists — the latter term, often 

 vaguely used, I always restrict to rocks which exhibit a 

 true foliation. In some schists the mineral constituents 

 are comparatively minute, in others they are of con- 

 siderable size. In the former case we may often venture 

 to affirm that the rock is a metamorphosed sediment ; in 

 the latter its original condition is a matter of conjecture. 

 Rocks of the former class often appear, to use no stronger 

 word, to lie above, and so to be less ancient than those 

 of the latter, and beneath that comes a coarser and more 

 massive series, in which granitoid rocks are common. In 

 these last foliation is often inconspicuous, and the rocks 

 in consequence are not markedly fissile. 



That these rocks are older than the Cambrian can often 

 be demonstrated. Sometimes it can even be proved that 

 their present distinctive character had been assumed 

 before the overlying Cambrian rocks were deposited. 

 Such rocks, then, we may confidently bring forward as 



types of the earth's foundation-stones. As the inscrip- 

 tions buried in the Euphrates valley tell us, the tongue of 

 Accad in the days prior to the coming of the Semite, so 

 these declare what then constituted the earth's crust. If 

 in such rocks we find any peculiarities of mineral com- 

 position or structure, these may legitimately be regarded 

 as distinctive. We have only to beware of mistaking for 

 original those which are secondary and subsequently 

 impressed. 



In other parts of the world we find rocks of like 

 characters with those above named, the age of which 

 cannot be so precisely fixed, though we can prove them 

 to be totally disconnected from and much older than the 

 earliest overlying stratum. To assert that these rocks 

 are contemporary with the others is obviously an hypo- 

 thesis which rests on the assumption that community of 

 structure has some relation to similarity of origin. I 

 am well aware that attempts have been made to discredit 

 this. But if we eliminate difficulties which are merely 

 sophistical — those, I mean, created by the use of 

 ambiguous or misleading terms — if we acknowledge 

 those due to our limited means of investigation, such as 

 that of distinguishing a rock crushed in situ from one 

 composed of transported fragments — in other words, of 

 separating in every case a superinduced from a primary 

 structure— and if we allow for others due to the limita- 

 tion of our instrumental and visual powers, I do not 

 hesitate, as the result of long and, I hope, careful work, 

 to assert that certain structures are very closely related 

 to the past history of a rock, and that in very many 

 instances our diagnosis of the cause from its effect is not 

 less worthy of confidence than that of an expert in 

 pathology or physiology. Resemblances of structures, 

 different in origin, do, no doubt, sometimes occur— 

 resemblances not seldom due to partial correspondence 

 in the environments ; but in regard to these it is our 

 duty to labour patiently till we succeed in distinguishing 

 them. The difficulty of the task does not justify us, 

 either in abandoning it in despair, or in sitting down, 

 after a few hasty observations, to fashion hypotheses 

 which have no better foundation than our own incom- 

 petence or idleness. 



As it is impossible in the time at my disposal to 

 demonstrate the proposition, I must assume what I 

 believe few, if any, competent workers will deny, that 

 certain structures are distinctive of rocks which have 

 solidified from a state of fusion under this or that environ- 

 ment; others are distinctive of sedimentary rocks ; others 

 again, whatever may be their significance, belong to 

 rocks of the so-called metamorphic group. I shall restrict 

 myself to indicating, by comparison with rock structures 

 of which the history is known, what inferences may be 

 drawn as to the history of the last-named rocks, which, 

 as I have already stated, are in some cases examples of 

 the earth's foundation-stones, while in others, if they are 

 not these, they are at any rate excellent imitations. 



Let us proceed tentatively. I will put the problem 

 before you, and we will try to feel our way towards a 

 solution. Our initial difficulty is to find examples of the 

 oldest rocks in which the original structures are still 

 unmodified. Commonly they are like palimpsests, where 

 the primitive character can only be discerned, at best 

 faintly, under the more recent inscription. Here, then, 

 is one of the best which I possess — a Laurentian gneiss 

 from Canada. Its structure is characteristic of the whole 

 group ; the crystals of mica or hornblende are well 

 defined, and commonly have a more or less parallel 



