Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/241

Rh men on THE EARTH.) present time, and is open to the obvious preliminary objection that it assumes the existing rate of change as the measure of past revolutions,——an assumption which may be entirely erro- neous, for the present may be-a period when all geological events march forward more slowly than they used to do. The argument proceeds on data partly of a physical and partly of an organic kind. (a) The physical evidence is derived from such facts as the observed rates at which the surface of a country is being lowered by rain and streams, and new sedimentary deposits are formed. These- facts will be more particularly dwelt upon in later portions of this article. If we assume that the land has been worn away, and that stratified deposits have been laid down nearly at the same rate as at present, then We must admit that the stratified portion of the crust of the earth must represent a very vast period of time. Dr Croll puts this period at not less, but possibly much more, than 60 million years. (L) On the other hand, human experience, so far as it goes, warrants the belief that changes in the organic world proceed with extreme slow- ness. Yet in the stratiﬁed rocks of the earth’s crust we have abundant proof that the whole fauna and ﬂora of the earth’s surface have passed through numerous cycles of re- volution,—speeies, genera, families, appearing and disappear- ing many times in succession. 011 any supposition it must be admitted that these vicissitudes iii the organic world can only have been effected with the lapse of vast periods of time, though no reliable standard seems to be available whereby these periods are to be measured. The argument from geological evidence is strongly in favour of an interval of probably not much less than 100 million years since the earliest form of life appeared upon the earth, and the oldest stratified rocks began to be laid down. 2. The argument from physics as to the age of our planet is based by Sir William Thomson upon three kinds of evi- (lence:—(l) the internal heat and rate of cooling of the earth ; ('2) the tidal retardation of the earth’s rotation ; and (3) the origin and age of the sun’s heat. (1.) Sir William Thomson, applying Fourier’s theory of thermal conductivity, poi11ted out some years ago (1862) that in the known rate of increase of temperature down- ward and beneath the surface, and the rate of loss of heat from the earth, we have a limit to the antiquity of the planet. He showed, from the data. available at the time, that the superﬁcial consolidation of the globe could not have occurred less than 20 million years ago, or the underground heat would have been greater than it is ; nor more than 400 million years ago, otherwise the underground temperature would have shown no sensible increase downwards. He admitted that very wide limits were necessary. In more reeentlydiscussiug the subject, he inclines rather towards the lower than the higher antiquity, but concludes that the limit, from a consideration of all the evidence, must be placed within some such period of past time as100millions of years.1 (2.) The argument from tidal retardation proceeds on the admitted fact that, owing to the friction of the tide- wave, the rotation of the earth is retarded, and is therefore much slower now than it must have been at one time. Sir William Thomson contends that had the globe become solid some ten thousand million years ago, or indeed any high- antiquity beyond 100 million years, the centrifugal force due to the more rapid rotation must have given the planet a very much greater polar ﬂattening than it actually pos- sesses. He admits, however, that, though 100 million years ago that force must have been about 3 per cent. greater than now, yet “nothing we know regarding the ﬁgure of the earth and the disposition of land and water would justify us in saying that a body consolidated when there was more 1 Trans. Roy. Soc. Edz'n., xxiii. 157; Trans. Gaol. Soc. Glasgow, iii. 25. GEOLOGY 227 centrifugal force by 3 per cent. than now might not now be in all respects like the earth, so far as we know it at present”? Professor Tait, in repeating this argument, con- cludes that, taken in connexion with the previous one, “ it probably reduces the possible period which can be allowed to geologists to something less than ten millions of years.”3 He does not state, however, on what grounds he so reduces the available period, nor does he notice the objection urged by Dr Croll that, rranting the gradual submergence of the polar lands owing to the slackened speed of rotation, the subaerial denudation of the rising equatorial land might well keep pace with the effects of the oceanic subsidence, so that we cannot infer from the present form of the earth what may have been its precise amount of polar compression at the time of solidiﬁeation.4 (3.) The third argument, based upon the age of the sun’s heat, is confessedly less reliable than the two previous ones. It proceeds upon calculations as to the amount of heat which would be available by the falling together of masses from space, which gave rise by their impact to our sun. The vagueness of the data on which this argument rests may be inferred from the fact that in one passage Professor Tait places the limit of time during which the sun has been illuminating the earth as, “ on the very highest computation, not more than about 15 or 20 millions of years,” while, in another sentence of the same volume, he admits that, “ by calculations in which there is no possibility of large error, this hypothesis [of the origin of the sun’s heat by the fall- ing together of masses of matter] is thoroughly competent to explain 100 millions of years solar radiation at the pre- sent rate, perhaps more.”5 One hundred millions of years is probably amply suﬁicient for all the requirements of geology. 1Il. Conrosrrrox on THE EAR'rH’s CRUST. MINERALS AND nocxs. The visible and accessible portion of the earth is formed of minerals and rocks. A mineral may be classiﬁed as an inorganic body distinguished by a more or less deﬁnite chemical composition, and usually a characteristic geo- metrical form. A rock is an aggregate mass, sometimes of one, more commonly of two or _more minerals. Upwards of 800 species of minerals and a vast number of varieties have been described. A very large proportion of these occur but rarely, and, though interesting and important to the mineralogist, do not demand the special attention of the geologist. While almost every mineral may be made to yield data of more or less geological signiﬁcance, only those which enter into the composition of rock masses, or which are of frequent occurrence as accessories there, require to be familiarly known by the student of geology. 1. Roek—Forming Minerals. The following are the more important minerals which enter into the composition of rocks :— Q-um-tz (SiO2) occurs either crystallized as rock-crystal, or non- crystalline as calcedony. In the former condition it an essential constituent of granite, felsite, and many other igneous rocks, as well as of sandstone and numerous aqueous rocks. The non-crys- tallized or colloid quartz is chieﬂy met with in cavities and ﬁssures of rock where it has been slowly deposited from aqueous solution. Numerous varieties of calcedony occur, as agate, carnelian, jasper, ﬂint, chert, Lydian-stone, &e. _ Fclspars (silicates of alumina, with potash, soda, or -lime) consti- tute the most abundant group of rock—forming minerals. _ For_the purposes of the petrographer they are conveniently divided into two scries—( 1) the Monoclinic or Orthoclase felspars (with cleavage angles of 90°), containing from 4 to 16 per cent. of potash and 3 Trans. Geol. Soc. Glasgow, iii. 16. 3 Recent Advances in Physical Science, p. 174. " Quart. Jour. Science, July 1877. 5 Op. cz't., pp. 153, 175.