August i8, 1904] 



NATURE 



We, visiting the scene of his labours more than thirty 

 years after he wrote these words, witness the realisation of 

 Sedgwick's hopes. The collection is not only worthy of 

 the University, but has become one of the finest in the 

 kingdom. It is housed in this magnificent memorial to the 

 name of Sedgwick, on the completion of which I offer for 

 myself, and I trust I may do so on behalf of this Section 

 also, hearty congratulations to the Woodwardian Professor 

 and his staff. Finally, I may remind you that at this 

 moment the Directorship of the Geological Survey and the 

 Presidential Chair of the Geological Society are held by 

 Cambridge men ; that the sister University has not dis- 

 dained to borrow from the same source ; and lastly, that 

 it is upon Cambridge chiefly that we have learned to depend 

 for recruiting the ranks of the Geological Survey, as proofs 

 that Cambridge has maintained her place among the fore- 

 most of the British schools of Geology. 



Though he had taken a leading part at former meetings 

 of the Association, Sedgwick's advanced age in 1862 

 necessitated rest, and this Section was deprived to a great 

 extent of the charm of his presence. It benefited, however, 

 in the fact that the Presidential Chair was occupied by one 

 of his most distinguished pupils. Jukes was one of those 

 men the extent of w-hose knowledge is not readily fathomed. 

 It has been my experience, and probably that of many others 

 in this room, to find that some conclusion, formed after 

 prolonged labour and perhaps fondly imagined to be new, 

 has been arrived at years before by one of the old geologists. 

 Such will be the experience of the man who follows Jukes's 

 footsteps. Turning to his Address given to this Section in 

 1862, we find much of what is now written about earth- 

 movement and earth-sculpture forestalled by him, with this 

 difference, however, that whereas the custom is growing of 

 using a phraseology which may sometimes be useful, but 

 IS generallv far from euphonious, and not always intelligible, 

 he states his arguments in plain, forcible English. 



It may raise a smile to find that Jukes thought it necessary 

 in 1862 to combat the view that deep and narrow valleys 

 had originated as fissures in the crust of the earth, and that 

 the Straits of Dover must have been formed in this way, 

 because the strata correspond on its two sides. But we 

 shall do well to remember that the smile will be at the 

 public opinion of that day, and not at Jukes himself. In 

 no branch of Geology have our views changed more than 

 in the recognition of the potency of the agents of denudation. 

 In 1862 it was necessary to present preliminary arguments 

 and to draw inferences which in 1904 may be taken as 

 granted. 



The evidences of the prodigious movements to which strata 

 "have been subjected, and of the extent to which denudation 

 has ensued, cannot fail to strike the most superficial observer. 

 Both mountain and plain present in varying degree proof 

 that sheets of sedimentarv material originally horizontal 

 are now folded and fractured. But after a momentary 

 interest aroused by some example more striking than usual, 

 glimpsed, it may be, from a train-window, the subject is 

 probably dismissed with an impression that such phenomena 

 are due to cataclysms of a past geological age, and have 

 little concern for the present inhabitants of the globe. These 

 ■stupendous disturbances, it might be argued, can only have 

 taken place under conditions different from those which 

 prevail now. We are familiar with mountain-ranges in 

 which their effects are conspicuous ; we have carried rail- 

 ways over or through them and have been troubled by no 

 cataclysmic movements of the strata. Apparently the rocks 

 have been fixed in their plicated condition, and are liable 

 to no further disturbance. Parts of the world, it is true, 

 are subject to earthquakes accompanied by fissuring and 

 slight displacement of the crust, but not even in earthquake 

 regions can we point to an example of such thrusting and 

 folding of the strata being actually in progress as have 

 taken place in the past. Nor, again, can volcanic activity 

 be appealed to, for some of the most highly disturbed regions 

 are devoid of igneous rocks. Volcanic eruptions are more 

 probably the effect than the cause of the disturbances of 

 the crust. Nowhere in the world therefore, it will be said, 

 can we see strata undergoing such violent treatment as 

 they have experienced in the past. How, then, can we 

 dispute the inference that the forces by which the folding 

 was produced have ceased to operate? 



Before accepting a conclusion which would amount to 



NO. 18 1 6, VOL. 70] 



admitting that the globe is moribund and that the forces 

 by which land has been differentiated from sea have ceased 

 to act, we shall do well to look more closely into the history 

 of the earth-movements to which any particular region has 

 been subjected. The investigation is one which calls for 

 the most intimate knowledge of the geological structure, 

 and, as time will admit of my dealing with a small area 

 only, I shall confine my observations to England and Wales, 

 selecting such facts as have been established beyond dispute. 



At the outset of the investigation we find reason to 

 conclude that the movements, so far as any one region is 

 concerned, have been intermittent. Evidence of this fact 

 is furnished wherever any considerable part of the geological 

 column is laid open to view. Sheets of sediment, aggre- 

 gating perhaps thousands of feet in thickness, have been 

 laid down in conformable sequence, all bearing evidence of 

 having been deposited in shallow seas. The inference is 

 inevitable that that period of sedimentation was a period of 

 uninterrupted subsidence. But sooner or later every such 

 period came to an end. Compression and upheaval took the 

 place of subsidence, and the strata lately deposited were 

 plicated and brought within the reach of denudation. 

 Illustrations of the recurrence of these movements abound, 

 and I need dwell no further upon them than to remark that 

 movements of subsidence and upheaval may be seen to have 

 alternated wherever opportunity is afforded for observation. 



On extending our observations we are led to infer that 

 the movements of the crust were developed regionally, not 

 universally. The areas of subsidence, for example, 

 evidenced by the marine formations, had their limits, 

 though those limits did not coincide with the shores of exist- 

 ing seas, nor has reason been found to believe that the 

 proportion of land to sea has varied greatly in past times. 

 The limits of the area affected by any one movement of 

 upheaval are more difficult to determine, but the effects 

 were manifested in the crumpling up of comparatively 

 narrow belts of country, and are easy of recognition. 



Further than this, we ascertain that the movements of 

 one region were not necessarily contemporaneous with those 

 of adjoining regions. The forces operating upon the crust 

 of the earth came into activity in different places at different 

 times, and, while some continental tracts have been but 

 little disturbed from early geological times, there are parts 

 of the globe which have been the scene, so to speak, of 

 almost ceaseless strife. Among the latter we may include 

 the British Isles. 



These are commonplaces of Geology, and I mention them 

 merely to emphasise the fact that the geological structure 

 of these islands is the result of movement superimposed 

 upon movement. Obviously, therefore, in order to gain a 

 comprehensive view of the operations which were in progress 

 in any one region during any one epoch, we have to find 

 some means of distinguishing the movements of that epoch 

 and of eliminating all which preceded or followed it. This, 

 briefly, is the problem which has engaged the attention of 

 geologists for many years past, and upon which I propose 

 to touch. 



The determination of the age of a disturbance is seldom 

 easy, and among the older Palaeozoic rocks is often im- 

 possible ; but at the close of the Carboniferous period, during 

 the great continental epoch which led to and followed upon 

 the deposition of the Coal Measures, there came into action 

 a set of movements of elevation and compression which 

 generally can be distinguished both from those which pre- 

 ceded them and from those which have been superimposed 

 upon them. The distinction depends upon the determination 

 of the age of the rocks affected by the movements. For 

 example, a movement by which the latest Carboniferous 

 rocks have been tilted from their original horizontal posi- 

 tion is obviously post-Carboniferous. On the other hand, 

 if Permian rocks lie undisturbed upon those tilted Carbon- 

 iferous rocks it is equally obvious that the movement was 

 pre-Permian. Now it happens that earth-movements of the 

 date alluded to were particularly active in the British Isles, 

 and played an important part in shaping the platform on 

 which the Permian and later rocks were laid down. 

 Though they have been more completely explored than others 

 in the working of coal, their further investigation is of the 

 greatest economic importance. I have attempted, therefore, 

 briefly to sketch out the principal lines along which earth- 

 movements of that age came into operation in England, 



