Correspondence — T. M. Rickman. 333 



VON ZITTEL'S HISTORY OF GEOLOGY. 



Sir, — I am one of those vvbo in consequence of your notice in the 

 April number at once sent for the translation of Von Zittel's 

 History of Geology, etc. I read it with avidity, and can endorse 

 all that was said about the book. Specially was I interested in 

 the masterly manner in which the subject of metamorphism, the 

 discussion on the Cambrian and Silurian systems, the JEozoon 

 Canadense, the North-West Highlands of Scotland, and the 

 unravelling of the Alpine strata were treated, with the various 

 points still open for investigation. 



There are, however, on p. 159, which contains statements as 

 to the diameter of the planets, and on p. 168, where the thickness 

 of the solid crust of the earth is dealt with, also on p. 300, where 

 the shortening of the earth's radius is mentioned, figures given 

 which I do not understand. The writer or the translator must have 

 had some modulus of dimension in mind different from any of those 

 stated in the text. It would be well before a second edition is pro- 

 duced that these points should be reconsidered. 



The following are the clauses remarked upon : — 



Page 159 : " Of the six planets that were known in early astrology, 

 Mercury is nearest the sun in position, and has itself a diameter of 

 648 miles ; Venus (diam. 1,613 miles) follows Mercury, then the 

 Earth (diam. 1,719 miles), then Mars (diam. 909 miles), Jupiter 

 (diam. 19,000 miles), and Saturn (diam. 16,675 miles). Herschel in 

 1780 discovered on the farther side of Saturn the planet Uranus with 

 a diameter of about 8,000 miles, and Leverrier in 1846 discovered 

 by mathematical calculation the outermost planet, Neptune, with 

 four and a half times the diameter of the Earth." 



The ordinary textbooks give — Mercury, 2,000 miles diameter ; 

 Venus, 7,600; Earth, 7,928; Mars, 4,430; Jupiter, 86,000 ; Saturn, 

 76,246 : Uranus, 32,000 ; Neptune, 35,000. 



Page 168 : " Hopkins calculated that the solid crust of the earth 

 had a thickness of about ^ or i of the earth's diameter, that is, at 

 least 172 to 215 geographical miles." 



Page 300 : " Delesse had calculated 1,340 metres as the amount 

 by which the earth's radius had already been shortened ; in other 

 words, the earth's crust in the course of the geological epochs had 

 approached the earth's centre by a distance about equal to the 

 height of Chimborazo or the Himalayas above sea-level." 



Thomas M. Eickman. 

 8, Montague Street, Russell Square, W.C. 

 Mai/ 31, 1902. 



THE LIMITS OF LEGITIMATE SPECULATION AT THE 

 GEOLOGICAL SOCIETY. 



SiR, — Early in 1900 I submitted to the Geological Society a short 

 paper on Bala Lake and the rivers of North Wales, in which 

 I attempted to show that the great valleys which run through 

 North Wales from north-east to south-west had probably been 



