498 



NA TURE 



[Skptk.mbkr 24, 1896 



the deep-water deposits of late Tertiary age found in the West 

 Indies indicate the occurrence of considerable uplift in that 

 region. But a great amount of work yet remains to be done 

 in this connection, especially concerning horizontal distortion of 

 masses of the earth's crust, owing to more rapid horizontal 

 advance of one portion than of another, during periods of move- 

 ment. Not until we gather together a large amount of informa- 

 tion derived from actual inspection of the rocks shall we be able 

 to frame satisfiiclory theories of earth-movement, and in the 

 meantime we are largely dependent upon the speculations of the 

 physicist, often founded upon very imperfect data, on which is 

 built an impo.sing superstructure of mathematical reasoning. 

 We have been told that our continents and ocean-basins have 

 been to a great extent permanent as regards position through 

 long geological ages ; we now reply by pointing to deep-sea 

 sediments of nearly all geological periods, which have been up- 

 lifted from the ocean-abysses to form portions of our continents ; 

 and as the result of study of the distribution of fossil organisms, 

 we can point almost as confidently to the sites of old continents 

 now sunk down into the ocean depths. It seems clear that our 

 knowledge of the causes of earth-movements is still in its 

 infancy, and that we must Ije content to wait awhile, until we 

 have further information at our disposal. 



Recent work has proved the intimate connection betwixt 

 earth-movement and the emissionand intrusion of igneous rocks, 

 and the study of igneous rocks has advanced beyond the petro- 

 graphical stage ; the rocks are now made to contribute their 

 share towards the history of different geological periods. The part 

 which volcanic action has played in the actual formation of the 

 earth's crust is well exemplified in Sir Archibald Geikie's Presi- 

 dential Addresses to the Geological Society, wherein he treats 

 of the former volcanic history of the British Isles. (Quart, 

 foiini. Geol. So(. , vols, xlvii. and xlviii. ) The way in which 

 extruded material contributes to the formation of sedimentary 

 masses has, perhaps, not been fully grasped by many writers, 

 who frequently seem to assume that deposition is a measure of 

 denudation, and vice vers,!, whereas deposition is only a 

 measure of denudation, and of the material which has been 

 ejected in a fragmental condition from the earth's interior, which 

 in some places forms a very considerable percentage of the total 

 amount of sediment. 



The intruded rocks also throw much light on past earth- 

 history, and I cannot give a better illustration of the valuable 

 information which ttiey may furnish to the stratigrajihical 

 geologist when rightly studied, than by referring to the excellent 

 and suggestive work by my colleague, Mr. Alfred Harker, on 

 the Bala \'olcanic Rocks of Carnarvonshire. (Sedg'd'ui; Essay 

 for iSSS : Camb. Univ. Press, 1889.) 



Perhaps the most striking instance of the effect which detailed 

 stratigraphical work has produced on geological thought is sup- 

 plied by the study of the crystalline schists. Our knowledge of 

 the great bulk of the rocks which enter into the formation of 

 a schistose complex is not very great, but the mode of produc- 

 tion of many of them is now well known, and the crude specu- 

 lations of some of the early geologists are now making way for 

 theories founded on careful and minute observations in the field 

 as well as in the laboratory. Recent work amongst the crystal- 

 line schists shows, furthermore, how careful we should be not to 

 assume that because we have got at the truth, we have therefore 

 ascertained the whole truth. We all remember how potent a 

 factor dynamic nietamorphism was supposed to be, owing to 

 discoveries made in the greatly disturbed rocks of Scotland and 

 Switzerland ; and the action of heat was almost ignored by 

 some writers, except as a minor factor, in the production of 

 metamorphic change. The latest studies amongst the foliated 

 rocks tend to show that heat does play a most important part in 

 the manufacture of schists. The detailed work of Mr. George 

 Barrow, in North-East Forfarshire ((?««?•/. _/ok>-«. Geo!. So,., vol. 

 xlix. (1S93) p. 330) has already thrown a flood of light upon 

 the origin of certain schists, and their connection with igneous 

 rocks, and geologists will look forward with eagerness to further 

 studies of the puzzling Highland rocks by this keen observer. 



The subject of former climatic conditions is one in which the 

 geologist has very largely depended upon followers of other 

 branches of science for light, and yet it is one peculiarly within 

 the domain of the stratigraphical geologist ; and information 

 which has already been furnished concerning former climatic 

 conditions, as the result of careful study of the strata, is probably 

 only an earnest of what is to follow when the specialist in 

 climatology pays attention to the records of the rocks, and 

 avoids the theories elaborated in the student's sanctum. The 



NO. 1404, VOL. 54] 



recognition of an Ice Age in Pleistocene limes at once proved 

 the fallacy of the supposition that there has been a gradual fall 

 in temperature throughout geological ages without any subse- 

 ()uent rise, and accordingly most theories which have been put 

 forward to account for former climatic change have been 

 advanced with special reference to the Glacial period or periods, 

 although there are many other interesting matters connected 

 with climate with which the geologist has to deal. Neverthe- 

 less, the occurrence of glacial periods is a matter of very great 

 interest, and one which has deservedly received much attention, 

 though the extremely plausible hypothesis of Croll, and the 

 clear manner in which it has been presented to general readers, 

 tended to throw other views into the shade, until quite recently, 

 when this hypothesis has been controverted from the point ot 

 view of the physicist. In the meantime considerable advance 

 has been made in our actual knowledge, and this year, probably 

 for the first time, and as the result of the masterly rc'siime of Prof. 

 Edgworth Uavid (" Evidences of Glacial Action in Australia in 

 Permo-Carboniferous Time," Quart. Joiirti. Geol. Soc, vol. lii. 

 p. 289), the bulk of British geologists are prepared to admit that 

 there has been more than one glacial period, and that the evi- 

 dence of glacial conditions in the southern hemisphere in 

 Permo-Carboniferous times is established. CroU's hypothesis ot 

 course requires the recurrence of glacial ]ieriods, but leaving 

 out of account arguinents not of a geological character, which 

 have been advanced against this hypothesis, the objection raised 

 by Messrs. Gray and Kendall (" The Cause of an Ice Age," Bril. 

 Assoc. Rep. (1892), p. 70S), that in the case of the Pleistocene 

 Ice .\ge " the cold conditions came on with extreme slowness, 

 the refrigerations being progressive from the Eocene period to 

 the climax," seems to nie to be a fatal one. At the same time, 

 rather than asking with the above writers " the aid ot 

 astronomers and physicists in the solution of" this problem, I 

 would direct the attention of stratigraphical geologists to it, 

 believing that, by steady accumulation of (acts, they are more 

 likely than any one el.se to furnish the true clue to the solution ol 

 the glacial problem. 



I have elsewhere called attention to marked changes in the 

 faunas of the sedimentary rocks when |iassing from lower to 

 higher levels, without the evidence of any apparent physical 

 break, or any apparent change in the physical conditions, so far 

 as can be judged from the lithological characters of the strata, 

 and have suggested that such sudden faunistic variations may be 

 due to climate. I refer to the matter as one which may well 

 occupy the attention of local observers. 



One of the most interesting points connected with climatic 

 conditions is that of the former general lateral distribution of 

 organisms, and its dependence upon the distribution of climatic 

 zones. The well-known work of the late Dr. Neumayr {" Ueber 

 klimatische Zonen wiihrend der Jura-und Kreidezeit," Denkschr. 

 der math.-iiatiirmisscn. C/asse der /;. /:. A/:ad. der IVisseii- 

 schaften, vol. xlvii. Vienna, 1883) has, in the opinion of many 

 geologists, established the existence of climatic zones whose 

 Ijoundaries ran practically parallel with the equator in Jurassic 

 and Cretaceous times, and the possible existence of similar 

 climatic zones in Palieozoic times has been elsewhere suggested ; 

 but it is very desirable that much more work should be done 

 upon this subject, and it can only be carried out by paying close 

 attention to the vertical and lateral distribution of organisms in 

 the stratified rocks. 



So far we have chiefly considered the importance of strati- 

 graphical geology in connection with the inorganic side of 

 nature. We now come to the bearing of detailed stratigraphical 

 work upon questions concerning the life of the globe, and here 

 the evidence furnished by the geologist particularly appeals to 

 the general educated public as well as to students of other 

 sciences. 



Attention has just been directed to the jirnbable importance 

 of former climatic changes in determining the distribution of 

 organisms, but the whole subject of the geographical distribution 

 of organisms during former geological periods, though it has 

 already received a con.siderable amount of attention, will doubt- 

 less have niuch further light thrown upon it as the result of 

 careful observations carried out amongst the stratified rocks. 



So long ago as 1853, Pictet laid it down as a pakeontological 

 law that " the geographical distribution of species found in the 

 strata svas mote extended than the range ol species of existing 

 faunas." One would naturally expect that at a time when tlie 

 diversity of animal organisation was not so great as it now is, 

 the species, having fewer enemies with which to cope, and on 

 the whole not too complex organisations to be affected by out- 



