September 21, 1893] 



NATURE 



487 



of fact are largely dependent upon, assistance from other de- 

 partments of knowledge ; but those who would help us should 

 bear in mind that the problems we have still to solve are ex- 

 tremely difficult and complex, so that if certain avenues of 

 thought are closed on insufficient grounds by arguments of the 

 validity of which we are unable to judge, but which we are 

 naturally disposed to take on trust, the difficulties of our task 

 may be greatly augmented and the progress of science seriously 

 retarded. So far as I can judge, there is no rj/rwW reason 

 why we should believe that any of the rocks we now see were 

 formed during the earlier stages of planetary evolution. We 

 are free to examine them in our own way, and to draw on the 

 bank of time to any extent that may seem necessary. 



For some years past the greater part of my time has been 

 devoted to a study of the composition and structure of rocks, 

 and it has occurred to me that I might, on the present occasion, 

 give expression to my views on the question as to whether the 

 present position of pelrographical science necessitates any im- 

 portant modification in the theoretical views introduced by the 

 uniformitarian geologists. Must we supplement the ideas of 

 Hutton and Lyell by any reference to primordial conditions 

 when we endeavour to realise the manner in which the rocks 

 we can see and handle were produced ? The question I propose 

 to consider is not whether some of these rocks may have been 

 formed under physical conditions different from those which now 

 exist — life is too short to make a discussion of geological pos- 

 sibilities a profitable pursuit — but whether the present state of 

 pelrographical science renders uniformitarianism untenable as a 

 working hypothesis ; and, if so, to what extent. There is 

 nothing original in what I am about to lay before you. All 

 that I propose to do is to select from the numerous facts and 

 more or less conflicting views, bearing on the question I have 

 stated, a few of those which appear to me to be of considerable 

 importance. 



The sedimentary rocks contain the history of life upon the 

 earth, and on this account, as well as on account of their ex- 

 tensive development at the surface, they have necessarily re- 

 ceived an amount of attention which is out of all proportion to 

 their importance as constituent portions of the planet. They 

 are, after all, only skin deep. If they were totally removed 

 from our globe its importance as a member of the solar system 

 would not be appreciably diminished. The general laws 

 governing the formation and deposition of these sediments have 

 been fairly well understood for a long time. Hutton, as 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 de- 

 nudation are mainly seen in the breaking 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 composition 

 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 success- 

 fully 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 in- 

 variably present in argillaceous deposits. Three, four, and 

 even five percent., 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 ordinary sur- 

 face 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 



1 J. W. Judcl, "Deposits of the Nile Delia," Proc. Royal Soc, vol. 

 xxxix. (1886), p, .ii3. 



NO. 1247, VOL. 48] 



to decomposition than the common white varieties, and its 

 geological life is, there.'ore, comparatively short.' Small 

 crystals and grains of zircon, rutile, ilmenite, cyanite, and tour- 

 maline are nearly indestructible, and occur as accessory con- 

 stituents in the finer-grained sandstones.- Garnet and stauroliie 

 also possess considerable powers of resistance, and are not un- 

 frequenlly 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 sedi- 

 ments. One of the oldest deposits in the British Isles is the 

 Torridon sandstone of the north-west of Scotland. The recent 

 discovery of Olenellus high up in the stratified rocks which un- 

 conformably 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 decom- 

 position did not act with the same force as now, or that they had 

 not been in operation for a sufficient length of time to eliminate 

 the felspar. A pure quartzose sand is probably never formed 

 by the direct denudation of a granitic or gneissose area. The 

 coarser .'■ediments thus produced contain in most, if not in all, 

 cases a considerable amount of felspar. But felspar is more 

 liable to decomposition by percolating waters when it occurs as 

 a constituent of grit than when present in the parent rock. 

 Silica may thus be liberated in a soluble form, and subsequently 

 deposited on the grains of quartz so as to give rise to secondary 

 crystalline faces, and kaolin may be produced as beautiful six- 

 sided tablets in the interstices of the grit. When the grit is in 

 its turn denuded the felspar is still further reduced in amount, 

 and a purer quartz-sand is formed. As the coarser detrital 

 material is used over and over again, thus measuring dif- 

 ferent periods of time like the sand in an hour-glass, the 

 felspar and other decomposable minerals are gradually elimin- 

 ated. The occurrence of a large amount of fresh felspar in the 

 Torridon sandstone might, I say, at first sight be thought to be 

 due to the great age of the reck. Any tendency to accept a 

 view of this kind is, however, at once checked when attention 

 is paid to the pebl)les in the coarser ccnglomeratic beds of the 

 same deposit. These consist largely of quartzite — a rock formed 

 by the consolidation of as pure a quartz-sand as any known to 

 exist in the later formations. We are therefore led to the 

 conclusion that the special features of the Torridon sandstone 

 are not a function of time, but of the local conditions under 

 which the rock was produced. 



A similar conclusion may be reached by considering other 

 types of sediment. When the stratified rocks of the different 

 geological periods represented in any limited area are compared 

 with each other certain marked differences may be observed, 

 but the different types formed in any one area at different times 

 can often be paralleled with the different types formed in 

 different areas at the same time, and also with those now form- 

 ing beneath the waters of rivers, lakes, and seas. Deep sea, 

 shallow water, littoral and terresttial deposits can be recognised 

 in the formations belonging to many geological periods, from 

 the most ancient to the moit recent ; and there is no evidence 

 that any of our sedimentary rocks carry us back to a lime when 

 the physical conditions of the planet were materially different 



r ''Notes on the Probable Origin of Some Slates," by W. Maynard 

 Hutchings, Ceol. Mas;., iSgo, p. 264. 



~ "Uebcr das Vorkommtn mikroskoplscher Ziikone und Titan- 

 Mincralien," von Dr. Hans Tliiirach, Vtrhandtd, phys.-mcdii. Geseltichaft 

 zu Wilrzburg, N.F. xviii. "On Zircons and other Minerals contained in 

 Sand," Allan B. Dick, Nature, vol. xxxvi. (1887), p. 91. See also "Mem. 

 Geo!. Survey," Geology of London, vol. i. p. 523. 



