July 6, 191 1] 



NATURE 



the higher cryptogams. The fifth period is illustrated 

 by Darwin and Pasteur. 



At the end of the book there is an interesting chrono- 

 logical table — a good lesson in itself. There is reason 

 for regret that students often take relatively little 

 in the historical development of the science 

 which they pursue. The excuse sometimes offered, 

 that they have no time for "historical studies," is 

 made impossible by a book like this, short and illu- 

 minating. It shows us, with singular success, how 

 "Biology, which in the sixteenth century sent out 

 only a few feeble shoots, has now become a mighty 

 tree with innumerable fruit-laden branches. The 

 vigour of its latest outgrowths encourages confident 

 hopes of future expansion." J. A. T. 



Catalogue of the Lepidoptera Phalcence in the British 

 Museum. Vol. x. Plates cxlviii-clxxiii. (London : 

 Printed bv order of the Trustees, 191 1.) Price 205. 

 Vol. x. of Sir George F. Hampson's great work on 

 the moths of the world was issued in November, 

 Kim, and was reviewed in Nature for February 23, 

 191 1 (p. 539). It was published in advance of the 

 plates, which were not quite completed, but which 

 appeared in May of the present year. They number 

 twenty-six, and on each plate thirty-two species are 

 figured, making a grand total of 842 species figured 

 out of 1222 described (except a few described in 

 vols. viii. and ix.) in vol. x., and if we add to this 

 number the 214 species figured in the text, we find 

 that only a few species are described and not figured, 

 and even of these most are recognisably figured else- 

 where. The enormous number of species of insects 

 makes this of great importance, and the close re- 

 semblance and frequent dull colouring of many species 

 often makes it difficult to point out their characters 

 bv description alone. With these excellent illustra- 

 tions it should be easy to identify most, if not all, the 

 species represented, and it is to be regretted that we 

 have not vet a sufficient series of illustrated works 

 on other orders of insects besides Lepidoptera. 



The Mechanism of Weaving. By Thomas W. Fox. 

 Pp. xxii + 604. Fourth edition. (London: Mac- 

 millan and Co., Ltd., 1911.) Price Js. 6d. net. 

 Since the first edition of this book was reviewed in 

 Nature on December 13, 1894 (vol. li., p. 149), 130 

 pages and twenty-six new illustrations have been 

 added. Some parts of the work have been rearranged 

 and others enlarged. The sections dealing with dob- 

 bies, Jacquards, figuring harnesses, card-cutting, pick- 

 ing, multiple shuttle boxes, letting off, taking up, 

 beating up, loom adjustments, and reeds have all been 

 extended; and those on gauze, lappets, and swivels 

 have been rewritten. Descriptions of terry weaving, 

 the automatic supplv of weft to looms, and warp stop 

 motions have been included in this edition, and also an 

 ind ;. 



Die Gruiidlehren der hoheren Mathematik. By Prof. 

 G. Helm. Pp. xvi + 420. (Leipzig: Akademische 

 Verlagsgesellschaft, 1910.) Price 13.40 marks. 

 This is much more like an English school class-book 

 than usually reaches us from Germany. Practically 

 it is a revision course of pure mathematics for what 

 we should call a degree standard. There are chapters 

 on differential and integral calculus, analytical plane 

 and solid geometry, differential equations, interpola- 

 tion, and the elementary theory of vectors. The 

 author has had in mind the requirements of technical 

 students, and his illustrations of theory are mainly of 

 a practical kind. Finally, there are nearly 400 

 diagrams, so that the "appeal to the eye" has not 

 been neglected. M. 



NO. 2i;5, VOL. 87] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Duration of Geological Time. 



There is at the present time a great discrepancy in the 

 numerical values given for geological time by the various 

 methods employed. Considering the period which has 

 elapsed since the commencement of the Cambrian, the 

 evidence afforded by the study of radio-active minerals 

 suggests that its length is of the order of 500 million years. 

 Arguments derived from the study of sedimentation give, 

 according to recent writers, a space of time for the same 

 period not exceeding 50 million years, and the method 

 based on the salinity of the ocean gives a similar figure. 

 It is with regard to sedimentation that I wish here to 

 make a few remarks. 



It is evident that at the present time the volume of 

 post-Eozoic sediments in existence is greater than ever it 

 has been, and also that it is slowly increasing at the 

 expense of the igneous and pre-Cambrian rocks now ex- 

 posed to denudation. Knowing the rate of this denuda- 

 tion and the total volume of these sediments, all of which 

 must necessarily have had their origin in igneous and 

 pre-Cambrian rocks, the period represented by the fossil- 

 iferous strata is given by a simple process of division. 



Situated as I am in Africa, without any geological 

 literature, I can only give the most approximate estimates 

 of the necessary factors with which to illustrate the method 

 and arrive at the required length of time. 



Sollas gives the thickness of the post-Eozoic sediments 

 as 250,000 feet, or approximately 50 miles. 



If we suppose all the sediments to be deposited at the 

 maximum rate, they will form a regular bed along the 

 continental shores for a distance of 35 miles^ seawards. 

 Taking 100,000 miles as the average shore line of the 

 Cambrian and subsequent periods, we arrive at the total 

 volume of sediments — 175 Xio 6 cubic miles. This spread 

 over the whole globe represents a thickness of seven- 

 eighths of a mile. 



The average rate of continental denudation is probably 

 1 foot in 5000 years, or 1 mile in 26 million years. The 

 continental" area is 56 million square miles, one quarter 

 of which is occupied by igneous and pre-Cambrian rocks, 

 i.e. 14 million square miles. The rate of denudation _ of 

 the latter is therefore 14 million cubic miles in 26 million 

 years. The time for all the sediments to collect at that 

 rate would be 325 million years. This figure is only an 

 indication of the order of the time elapsed. If requires to 

 be corrected for the following factors : — 



A. Factors tending to decrease the estimate. 



(1) Exposed surface of igneous and pre-Cambrian rocks 

 may have been greater in former ages. 



(2) Marine denudation of igneous and pre-Cambrian 

 coasts. 



(3) Greater density of igneous rocks than of sediments. 



B. Factors tending to increase the estimate. 



(1) Average land area has probably been about o-S that 

 of to-day. 



(2) Average continental mean height has probably been 

 below that of to-day. 



(3) Recent glaciation has laid bare many pre-Cambrian 

 areas. 



(4) Present is a volcanic period, increasing the weather- 

 ing capacity of rain. 



(5) Present climates are of maximum variability. 



(6) Parts of land area are subject to deposition, i.e. 

 negative denudation. 



(7) Unconformities not represented by sediments else- 

 where. 



(S) Some sediments at great depths may have become 

 igneous rocks. 



It is hoped that, with the increase of accurate quantita- 

 tive knowledge of former conditions, and of the pro< 



