736 KEPOET — 1893. 



understood for a long time. Hutton, aa we have already seen, clearly realised that 

 the land is always wasting away, and that the materials are accumulating on the 

 beds of rivers, lakes, and seas. The chemical effects of denudation are mainly seen 

 in the hreakiug up of certain silicates and the separation of their constituents into 

 those which are soluble and those which are insoluble under surface conditions. 

 The mechanical effects are seen in the disintegration of rocks, and this may, under 

 certain circumstances, take place without the decomposition of their component 

 minerals.' Quartz and the aluminous silicates, which enter largely into the compo- 

 sition of shales and clays, are two of the most important insoluble constituents. It 

 must be remembered, however, that felspars often possess considerable powers of 

 resistance, and rocks which contain them may be broken up without complete or 

 anything like complete decomposition of these minerals. Orthoclase, microcline, 

 and oligoclase are the varieties which most successfully resist decomposition ; and, 

 as a natural consequence, occur most abundantly in sedimentary deposits. It is 

 commonly stated that when felspars are attacked the general effect is to reduce 

 them to a fine powder, composed of a hydrated silicate of alumina, and to remove 

 the alkalies, lime, and a portion of the silica. But, as Dr. Sterry Hunt has so 

 frequently urged, the removal of alkalies is imperfect, for they are almost invariably 

 present in argillaceous deposits. Three, four, and even five per cent., consisting 

 mainly of potash, may frequently be found. This alkali appears to be present in 

 micaceous minerals, which are often produced, as very minute scales, during the 

 decomposition of felspars. White mica, whether formed in this way or as a product 

 of igneous or metamorphic action, possesses great powers of resistance to the ordi- 

 nary surface agencies of decomposition, and so may be used over and over again 

 in the making of sedimentary deposits. Brown mica is also frequently separated 

 from granite and other rocks, and deposited as a constituent of sediments ; but it is 

 far more liable to decomposition than the common white varieties, and its geological 

 life is, therefore, comparatively short." Small crystals and grains of zircon, rutile, 

 ilmenite, cyanite, and tourmaline are nearly indestructible, and occur as accessory 

 constituents in the finer-grained sandstones.^ Garnet and staurolite also possess 

 considerable powers of resistance, and are not unfrequently present in the same 

 deposits. If we except the last two minerals and a few others, such as epidote, the 

 silicates containing lime, iron, and magnesia are, as a rule, decomposed by surface 

 agencies and the bases removed in solution ; augite, enstatite, hornblende, and 

 lime-felspars are extremely rare as constituents of ordinary sediments. 



The insoluble constituents resulting from the waste of land surfaces are deposited 

 as gravel, sand, and mud ; the soluble constituents become separated as solid bodies 

 by evaporation of the water in inland seas and lagoons, by chemical action, and by 

 organic life. They are deposited as carbonates, sulphates, chlorides, and sometimes, 

 as in the case of iron and manganese, as oxides. The soluble silica may be deposited 

 in the opaline condition by the action of sponges, radiolaria, and diatoms, or as 

 sinter. 



The question that we have now to consider is whether there is any marked 

 difference between ancient and modern sediments. One of the oldest deposits in 

 the British Isles is the Torridon sandstone of the north-west of Scotland. The 

 recent discovery of Olenellxis high up in the stratified rocks which unconformably 

 overlie this deposit has placed its pre-Cambrian age beyond all doubt. Now this 

 formation is mainly composed of quartz and felspar, at least in its upper part, and 

 the latter mineral is both abundant and very slightly altered. One is naturally 

 tempted, at first sight, to associate the freshness of the felspar with the great age 

 of the rock — to assume either that the sand was formed at a time when the chemical 

 agents of decomposition did not act with the same force as now, or that they had 



' J. W. Judd, ' Deposits of the Nile Delta,' Prcc. Royal Soc, vol. xxxix. 1886, p. 213. 



^ ' Notes on the Probable Origin of some Slates,' by W. Maynard Hutchings, 

 Geol. Mag., 1890, p. 261. 



' ' Ueber das Vorkommen mikroskopischer Zirkone und Titan-Mineralien,' von 

 Dr. Hans Thiiracb, Vcrhandl. d. phys.-medic. Gesellscliaft zn Wilrzhurg, N.F. xviii. 

 ' On Zircons and other Minerals contained in Sand,' Allan B. Dick, Nature, \o\. xxxvi. 

 (188'^), p. 01. See also ' Mem. Geol. Survey," Geology of Luncion, vol. i. p. 523. 



