24 SCIENCE PROGRESS 



the lead is attached to carbon atoms by fewer than four 

 valencies ; the new compound, which is prepared by the addi- 

 tion of finely powdered lead chloride to an ethereal solution 

 of magnesium p-2-xylyl bromide, gives molecular weight deter- 

 minations agreeing with the formula [(C8H9)3Pb]2. It is 

 assumed that the lead atoms are united in much the same 

 manner as the carbon atoms of the triaryl methyls. The 

 substance is readily attacked by bromine, giving lead tri-p-2- 

 xylyl bromide ; although in solution it is rapidly and com- 

 pletely decomposed on exposure to light, the solid is remark- 

 ably stable towards air. 



GEOLOGY. By G. W. Tyrrell, A.R.C.Sc. F.G.S.. University, Glasgow. 



Economic Geology. — During 191 8 the Geological Survey of the 

 United Kingdom was still almost exclusively occupied with 

 economic work of military or commercial importance {Sum- 

 mary of Progress for 191 8, 191 9, p. 70). In the summary of 

 progress the lead- and zinc-mines of Scotland are specially 

 treated. There is also an interesting account of the bauxitic 

 fireclay of North Ayrshire, which has proved of great value 

 as a refractory. This occurs in the Millstone Grit, and over- 

 lies a series of decomposed basic lavas. Its intimate associa- 

 tion with beds rich in iron and resembling laterite suggests 

 that its origin was similar to that of bauxite. The compact, 

 non-porous character of the clay may be due to subsequent 

 silicification, but it may originally have been deposited in its 

 present form. 



An important monograph, by E. C. Harder, on iron-depositing 

 bacteria and their geological relations, has been published by 

 the United States Geological Survey {Prof. Paper, 113, 191 9, 

 p. 89). Harder shows that in addition to the iron bacteria 

 proper, it is probable that many of the common bacteria of 

 soil and water cause the precipitation of ferric hydroxide from 

 solutions containing iron salts of organic acids. The second 

 part of the work contains an excellent review of sedimentary 

 iron ores and the part played by organic agencies, including 

 bacteria, in their deposition. 



Several reports have recently been issued on the iron-ore 

 deposits of the United Kingdom (F. H. Hatch, " Recent Iron 

 Ore Developments in the United Kingdom," Geol. Mag. 1919, 

 pp. 387-97 ; B. Smith, " Iron Ores : Haematites of West Cumber- 

 land, Lancashire, and the Lake District," Mem. Geol. Surv., 

 Special Reports on the Mineral Resources of Great Britain, 19 19, 

 8, p. 182 ; T. F. Sibly, "The Haematites of the Forest of Dean 

 and South Wales," ibid., 10, p. 93 ; T. C. Cantrill, et alii, "Sundry 

 Unbedded Ores of Durham, East Cumberland, North Wales, 



