NATURE 



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THURSDAY, AUGUST 15, 1907. 



CORSISH GEOLOGY. 

 I'hc Gcolugv of Falmouth and Truro, and of llic 

 Mining District of Camborne and Redruth. By 

 J. B. Hill and D. A. MacAlister, with Petrographical 

 Notes bv J. S. Flett. Memoirs of the Geological 

 Survey. Pp. x + 335; with 65 illustrations and 24 

 plates. (London : Wyman and Sons, Ltd., 1906.) 

 Price ~s. 6d. 



OF the new series one-inch map of England and 

 Wales, few sheets are of greater interest than No. 

 352 (colour-printed, price is. 6d.), which represents that 

 portion of western Cornwall that contains the oldest 

 and richest mines in the county; and the value of the 

 map is greatly enhanced by the admirable explanatory 

 memoir, which contains a description of the older slaty 

 rocks, the granites, elvans, and greenstones of this 

 area, together with particulars of the metalliferous 

 veins, .\ttention is given to the genesis of the ores, 

 and to their mode of occurrence, to the methods of 

 dressing the tin ore, and to the mineral output. In 

 short, the description is much more detailed than is 

 usual in a Geological Survey sheet explanation. 



The original geological survey was carried out by 

 De la Beche, whose results were published in 1839. 

 New editions of the maps were published in 1S66, with 

 additional lodes inserted by Sir Warington Smyth. A 

 comparison of the new map with the older one shows 

 that the additions that have been made are of great 

 importance. The area formerly represented as Devon- 

 ian has been subdivided into Lower Devonian and 

 three subdivisions, based on lithological characters, 

 designated by the terms Portscatho, Falmouth, and 

 Mvlor. These three subdivisions, together with the 

 \'eryan beds, formerly classed as Lower Silurian, are 

 shown to be Lower Pateozoic. The grits of Gram- 

 pound and Probus, representing the base of the 

 Devonian rocks, are taken as the equivalents of the 

 conglomerate on the Helford River (one-inch map, 



359)- 



The igneous rocks are divided by Mr. Hill into four 

 groups : (i) the greenstones, the oldest, which are epi- 

 diorites derived from dolerites and basalts ; (2) the 

 granite, the dominant representative, with which the 

 mineral products of the area are intimately associated ; 



(3) the elvans, which represent the dyke phase of 

 granite, and were intruded after its consolidation ; and 



(4) the mica traps, the age of which is uncertain. 

 Chapters are devoted to a description of the contact 

 metamorphism effected by the granite and of the 

 faults. The fractures are the product of the later Car- 

 boniferous period, and have a trend of about E.X.E. 

 Many of them occur in the mineral areas, where they 

 form the home of the lodes. They are traversed by a 

 second set of faults, of earl\- Tertiary age, which are 

 known to the miner as cross-courses. The Pliocene 

 deposits receive careful attention, the Polcrebo gravels 

 appearing to represent a patch of the Pliocene deposits 

 which formerly spread over the killas platform of this 

 area, and are represented at St. Erth, St. .Agnes, and 

 Crousa Downs in adjacent districts. With the ex- 



NO. 1972, VOL. 76] 



ception of these gravels, this area affords no evidence 

 of stratified deposits from the Palseozoic period until 

 the Pleistocene, which is represented by the raised 

 beaches underlying the Head, that is probably a 

 product of the Glacial age. 



The second part of the work, which is written by 

 Mr. MacAlister, deals with the mining industry of the 

 district. Modern theories of the genesis of ore deposits 

 are applied to the Cornish tin and copper ores, and de- 

 tailed descriptions of the mines are given, illustrated by 

 more than fifty sketches. Statistics of production since 

 1845 are set forth in detail, and ten pages are devoted 

 to a description of the tin-ore dressing' processes 

 employed in the Camborne area. Products from the 

 various operations have been subjected to microscopic 

 e.Kamination, the results showing that grains of cassit- 

 erite which are less than the average size of the 

 grains in the concentrates tend to be lost in the tail- 

 ings of subsequent operations. The dressing oper- 

 ations are elucidated by means of excellent reproduc- 

 tions of photographs. 



On the whole, the work is a very creditable produc- 

 tion. It forms an excellent introduction to the geology 

 of Cornwall; and it is satisfactory to note that the 

 price at which it is published is not such as to preclude 

 its use by students of economic geology. 



MODERN INTRODUCTIONS TO PHYSICAL 

 THEORIES. 



(i) Elcktromagnctische Schwingungcn und Wcllen. 

 By Dr. Josef Ritter von Geitler. Pp. viii+154. 

 (Brunswick : Fried. Vieweg und Sohn, 1905.) 

 Price 4.50 marks. 



(2) Theorie der Elektridiat. By Dr. A. Foppl and 

 Dr. M. Abraham. Vol. i., pp. xviii-l-443; vol. ii., 

 pp. x + 404. (Leipzig: B. G. Teubner, 1904 and 

 1905-) 



(3) Leber den gcgen-Lvartigen Stand der Frage nacli 

 einer mechanischen Erkldrung der elektrischen 

 Erscheinungen. Bv Dr. Hans Witte. Pp. xii + 

 232. (Berlin : E. Ebering, 1906.) Price 7.50 marks. 



(4) Die Fortschritte der hinetischen Gastheorie. By 

 Dr. G. Jager. Pp. 1x4-119. (Brunswick: F. 

 Vieweg' und Sohn, 1906.) Price 3.50 marks. 



(5) .4)1 Elementary Treatise on Theoretical Mechanics. 

 'By J. H. Jeans, F.R.S. Pp. viii + 364. (Boston 



and London : Ginn and Co., n.d.) Price 10.S. 6d. 



A COMPARISON of text-books of the last year or 

 two with those published, say, a quarter of a 

 century ago affords a clear and definite measure of the 

 great changes which have occurred during the interval 

 in our commonly accepted physical theories. .\11 the 

 books now before us reflect these changes in a greater 

 or less degree, but we take Dr. von Geitler 's manual 

 as our first example because the author has closely 

 followed the historic order of development in leading 

 his readers from the simple early notions of action at 

 a distance to the modern theories of electrical oscilla- 

 tions and their applications to wireless telegraphy. 



Starting with Newton's discovery of the law of 

 gravitation, the reader is led up to Faraday's re- 

 searches on the electromagnetic field, and he first 



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