June 19, 1913] 



NATURE 



for example, on the Cretaceous ; Wethered on the 

 Carboniferous; Hutchings on slates and other 

 rocks, Millard Reade and Thomas on the Trias ; 

 Cayeaux on many formations, to mention a few 

 only of the workers. Yet, on the whole, the peno- 

 logical problems of sedimentary rocks have not at- 

 tracted the attention of geologists with anything 

 like the same force as those presented by the 

 igneous rocks. 



The reasons for this partial neglect are not far 

 to seek. Most active, perhaps, has been the in- 

 fluence of the "path of least resistance." It has 

 been much easier to make a brave show for a 

 given expenditure of time and care on the petro- 

 logy of igneous than on that of sedimentary rocks. 

 Though many sediments, especially when much 

 metamorphosed, may be studied in thin sections, 

 the examination of less compact sediments requires 

 the handling of a considerable bulk of material 

 by somewhat tedious processes, as well as the 

 application of a sound knowledge of the methods 

 of mineral identification. Thus we find the fresher 

 sediments passed over in favour of their most 

 highly altered representatives, which have received 

 a very large share of attention. 



It is therefore a matter for congratulation that 

 we have now, for the first time, a book (1) devoted 

 to the petrology of the sedimentary rocks, and, 

 further, that it is a companion to Hatch's "Text- 

 book of Petrology," which has a well-earned re- 

 putation for lucidity of treatment. 



The book is divided into two parts, the first 

 dealing with the mode of formation and materials 

 of raw sediments (employing this term in its 

 widest sense, to include all but rocks of igneous 

 origin), the second with their metamorphosed 

 equivalents. 



In the former the subject-matter is treated on 

 familiar lines under the heads deposition in 

 general, fragmental deposits, chemical deposits, 

 organic deposits. The clastic sediments are 

 graded according to the dimensions of their con- 

 stituents, e.g. boulder and scree deposits, above 

 100 mm. diameter; gravel, between 2' 5 and 100 

 mm. ; sand, between 0^05 and 2-5 mm. ; mud and 

 dust, below o'05 mm. 



It seems a little unreasonable to describe the 

 foraminiferal, pteropod, diatom, and radiolarian 

 oozes with the fragmental deposits, while shell 

 and coral formations appear in the organic 

 deposits. 



Here and there, as is usual in text-books, diffi- 

 cult ground is lightly tripped over; the explana- 

 tion of the formation of "iron-pan " is not all that 

 could be desired. 



From fresh sediments the authors pass on 

 NO. 2277, VOL. 91] 



395 



to what they call the metamorphic derivatives, and 

 follow Van Hise and many modern writers in the 

 assumption that metamorphism must be taken to 

 connote any change in the constitution of any 

 kind of rock, through whatever agency. This 

 is logical, no doubt, but "horse sense" is some- 

 times better than any amount of logic; by being 

 so precise about the literal meaning of the word, 

 its old-fashioned and quite useful significance has 

 been destroyed. By their acceptance of this ex- 

 tended usage of "metamorphism," the authors 

 stultify in a measure their own classification. 

 Why should they trouble to separate the "sedi- 

 ments" from their "metamorphic derivatives" 

 when they must be aware that the former are 

 suffering, from the earliest moments of their forma- 

 tion, those very changes of constitution that pro- 

 duce the so-called metamorphism. There can be 

 no unmetamorphosed rocks if metamorphism is 

 what the authors say it is. However, coming to 

 details, we find this subject treated under the 

 heads contact metamorphism, regional meta- 

 morphism, cementation, metasomatism, and 

 weathering. The "zone of cementation" is said 

 to be permanently saturated with water-solutions, 

 but we would point out that cementation may 

 take place in the "zone of weathering," and, 

 indeed, numerous examples are given in the 

 book. 



This volume contains little that cannot be ob- 

 tained by the perusal of the larger general text- 

 books ; none the less, it will be a convenience 

 to students to have the information in this handy 

 form. In a new edition, which we hope will soon 

 be required, we should like to see more illustra- 

 j tions of typical rocks as good as those in the 

 ; present work; photomicrographs of metasomatic 

 and other structures in limestones and series to 

 I illustrate progressive alteration in contact and 

 regional metamorphism are specially to be desired. 

 The value of this volume is greatly enhanced by 

 the admirable appendix on the systematic exami- 

 nation of loose detrital sediments by Mr. 

 Crook. 



(2) Prof. Ries is as well known in this country 

 as in the United States for his numerous and 

 valuable works on clays. There is little that is 

 new in his book on building stones and clay-pro- 

 ducts, but it is well produced, and compiled by 

 an experienced hand ; it will doubtless be appreci- 

 ated by the United States architects, for whom it 

 has been written. The viewpoint throughout is 

 entirely American, as the following extract from 

 the glossary will illustrate: "Forest-marble, an 

 argillaceous limestone in which the colouring- 

 matter is so disposed as to resemble forests." 



