NATURE 



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THURSDAY, FEBRUARY 2, 1899. 



THE RECORDS OF THE ROCKS. 

 TJic Principles of Stratigraphical Geology. By J. E. 

 AFarr, M.A., F.R.S. Pp. 304. (Cambridge : Univer- 

 sity Press, 1898.) 



C"' EOLOGY is admittedly a complex science, and 

 J Mr. Marr, no doubt rightly, speaks of it as one of 

 the uncertain sciences. It is well for the student to be 

 able clearly to discriminate between that which is ascer- 

 tained and that which is inferred : and even individual 

 knowledge may be dangerous when the limits of it are 

 not clearly realised. The task of the stratigraphical 

 geologist is, as the author points out, to establish the 

 order of succession of the strata and to ascertain the 

 conditions which existed during their deposition. 



Possessed of a good general knowledge of the elements 

 of geology and pakeontology, the enthusiastic student 

 will desire to engage in original observations in the 

 field, and in the absence of the Cambridge University 

 Lecturer in Geology, he will do well to carry this book 

 with him. The chapter on the growth and progress of 

 stratigraphical geology shows how much was accotn- 

 plished in early days by traverses and sketch-maps, how 

 the broad outlines were to some extent filled in on one- 

 inch maps by subsequent observers, and how necessary 

 it is nowadays to labour in greater detail and express 

 the results of field-work on maps on the scale of six 

 inches or even twenty-five inches to a mile. Experience 

 in geological surveying is the best basis for further work 

 — the student will then understand the structure of the 

 ground, the local sequence of the strata and the nature 

 of their organic remains. Even in geological mapping 

 the uncertainties of the science, which Mr. Marr takes 

 care to point out, are nowhere absent. 



In considering the terms applied to sedimentary rocks, 

 .tnd the varying nature of the rocks themselves, we 

 may feel that there is a want of precision in our language 

 and in our geological boundary-lines ; but the geologist 

 soon learns that harmony and order are everywhere 

 apparent amid the gradual changes of scene and life 

 and climate which the strata reveal, and that their 

 continuity is only locally interrupted. With regard to 

 uniformitarianism the author rightly maintains that it is 

 unphilosophical to hold that the agents which are in 

 operation to-day are similar both in kind and intensity 

 to those which were at work in past times. At the best 

 our information is too incomplete to allow of dogmatism ; 

 and the student should be prepared "to consider that 

 the more active operation of agents, even in times of 

 which he has definite knowledge, may have produced 

 eftects which he should be prepared to discover." 



Every year the labours of the student become more 

 detailed. From the broad groupings of the old masters 

 we have now to consider minute subdivisions in the 

 strata, and the paL-eontological stages or zones which 

 mark the succession of life. These detailed studies are 

 all important when we seek to make out the chronology 

 of the earth in general, but there is no doubt that much 

 ol the minute work m connection with the succession 

 NO. 1527, VOL. 59] 



and evolution of species is more of a biological than a 

 geological study. The geologist is mainly concerned 

 with the natural history of successive periods, with the 

 character of the formations, the conditions and climate 

 which they indicate, and the movements to which they 

 have been subjected. 



In perusing this volume the student will find many a 

 useful hint with regard to the superposition of strata, 

 the effects of folds and faults, the included organisms 

 whether original or derived, and the simulation of 

 structures organic and inorganic. In considering the 

 somewhat vexed subject of contemporaneity, the author 

 is careful to point out that when we find the same fauna 

 in different localities it is justifiably assumed that the 

 species did not originate simultaneously in the two 

 areas. Nevertheless, "everything depends on the time 

 taken for migration as compared with the period of 

 existence of the fauna." Thus we may " speak of the 

 strata as contemporaneous, just as an historian would 

 rightly speak of events in the same way which occurred 

 upon the same afternoon, though one might have 

 happened an hour before the other." The student will 

 do well to bear in mind these remarks, for a few hours 

 in our lives may be as a hundred, or even a thousand, 

 years in the life-history of a zone. 



Again, in reference to the recurrence of species, Mr. 

 Marr observes that instances are by no means rare, but 

 "that the whole fauna does not disappear for a time 

 and then reappear, but only a few out of the many forms 

 which compose it." Some remarks are naturally made 

 on homotaxis, and to Huxley's assertion that a Devonian 

 fauna and flora in the British Islands may have been 

 contemporaneous with Silurian life in North America 

 and with a Carboniferous fauna and flora in Africa. 

 Subsequent researches have in no degree lent support 

 to this notion, and we may be content to speak of 

 practical contemporaneity without implying that absolute 

 synchronism which it would be impossible to establish 

 in comparing formations far apart. It is curious to note 

 on p. 19, that Mr. Marr speaks of the Devonian system 

 being finally placed "upon a secure basis," while on 

 p. 184 he admits that the Devonian question is not 

 settled. The fact is that the characteristic Middle 

 Devonian fauna is distinct, but we do not yet know to 

 what extent the Lower and Upper Devonian strata in 

 this country include horizons elsewhere grouped as 

 Silurian or Carboniferous. 



.After giving some account of the conditions under 

 which strata are formed, the author reviews generally 

 the successive stratified formations, both in this country 

 and abroad, without entering into much detail. With 

 regard to terms, we would not say that " Primary has 

 been definitely abandoned." It is used by Sir A. Geikie 

 in his "Text-book of Geology," by Prestwich in his 

 "Geology," vol. ii., and by Lapworth in his "Interme- 

 diate Text-book of Geology" (1899). The term Permo- 

 Carboniferous is useful as a temporary name for a group 

 to which much attention is being given; but we would 

 rather use Holocene than " The Forest Period " for the 

 deposits often spoken of as Recent, and which succeed 

 the Pleistocene. We have said enough, however, to indi- 

 cate that the work will prove exceedingly useful to the 



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