480 Correspondence— Geological Time, 



and other rocks used in the arts and manufactures, with engineering 

 geology and soils. In ascertaining the mode of occurrence and 

 extent of various deposits of economic importance, a scientific 

 knowledge of the structure of the ground is essential. In metalli- 

 ferous regions it is needful to know the character of the plications 

 as well as the nature of the rocks. The width of lodes may vary 

 with rocks of different texture, and cavities filled with mineral 

 matter may be expected at particular points in flexured rocks. 



After giving some account of unstratified ore-deposits, the author 

 briefly refers in chapter vii to peat, lignite, coal, and petroleum, 

 and also to phosphates, rock-salt, gypsum, and cobalt. We cannot 

 say that the author's treatment of his subject is systematic. In 

 chapter viii we are told something about prospecting and developing, 

 and the recognition of minerals, and also about quarrying and 

 mining. Thence we pass on, in chapter ix, to building and 

 ornamental stones. Here the author commences with a general 

 account of igneous rocks, of dykes and sills and laccolites, and 

 gives a short glossary of terms applied to rock-structures, a classi- 

 fication of igneous rocks, and a list of their chief rock-forming 

 minerals. He then deals briefly with causes of weathering in 

 building stones, a subject discussed subsequently in several chapters. 

 In chapter xiii we have accounts of lithographic stone, lime and 

 cement, brick and pottery claj's, fire-clays, sands, grindstones, bath 

 bricks, pigments, gems, and artificial stone. Fuller's earth is briefly 

 mentioned, but that of Nuffield and Woburn should have been 

 referred to Lower Greensand and not Lower Oolite. 



Chapter xiv contains an account of water-bearing strata and 

 water-supply, with some useful diagrams, and two that are far 

 from intelligible (figs. 147 and 149). Here the stratigraphical 

 information is too meagre to be of much practical use. The 

 references to impounding reservoirs in chapter xv are again too 

 meagre to be of service. The concluding chapter deals with soils, 

 and there is an appendix entitled " Simple rough methods for the 

 determination of minerals and rocks." It would be well, as the 

 author suggests, that the enquirer commences " by experimenting 

 with known specimens." 



coi2,iaESi=oiNrnD:BisrG:E. 



GEOLOGICAL TIME. 



SiE, — All geologists will thank Sir A. Geikie for his admirable 

 address to Section C at Dover this year. May I add another 

 sentence bearing on "geological time," from Huxley's address in 

 February, 1869 (Q.J.G.S., vol. xxv, p. l). He writes : 

 " Mathematics may be compared to a mill of exquisite workmanship, 

 which grinds you stuff of any degree of fineness ; but nevertheless, 

 what ynu get out depends on what you put in ; and as the grandest 

 mill in the world will not extract wheat-flour from peascods, so 

 pages of formulge will not get a definite result out of loose 

 data."— F. G. S. 



TuRNHAM Green. 



