1 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



periods by forms which are subject to limited modification in each 

 period, rises naturally to the mind as best expressing the actual 

 relations of the world of life — the idea of one great design, reaching 

 through the immensity of time, essentially limited in its expression 

 by selected combinations, but within these limits admitting of a vast 

 variety of adjustments to varying local conditions :-— 



" Quoniam minui rem quamque videmus, 



Et quasi longinquo fluere omnia cernimus sevo, 



Ex oculisque vetustatem subducere nostris ; 



Quum tamen incolumis videatur summa manere ; 



§ Geological Time. 



The earth, then, has a History — more complete in the sequence 

 and more precise in the knowledge of events than is usually found 

 in the history of any far-descended races of men. We cannot, in- 

 deed, marshal the events under letters which shall stand for anno 

 mundi ; we cannot compute their dates by years and days, by eclipses 

 and conjunctions ; and, in this sense, the ancient and the modern 

 history of the world cannot be measured by the same Chronology., 

 We cannot know the antiquity (that is to say, the date in solar time) 

 of any one ancient horizon of life, or great system of natural 

 agencies. But we are not debarred from computing the relative 

 lapse of time among the ancient deposits. The laws of Nature are 

 the same to-day as in the earliest time, however much the conditions 

 may have varied, and the ratio of effects to time may have varied 

 also. If we choose among the conditions those which are least vari- 

 able, as the mechanical action of water put in motion by declivity 

 of ground or tidal agitation, or wind excited by differences of tem- 

 perature and changing state of vapour — in a word, the atmospheric, 

 fluviatile, and oceanic agencies by which the earth's surface is wasted 

 in one part and modified by aggregation elsewhere, — we may em- 

 ploy the mean result of these actions as equivalent to a unit of time. 

 And, though it should appear that the conditions assumed as constant 

 were really variable, this variation may have been (or rather must 

 have been) according to a law of gradual change to greater or less, 

 which may become sufficiently known to allow of a probable cor- 

 rection. 



The accumulation of strata of a sedimentary character, as sand- 

 stones and clays, is a result of the kind here looked for, and is ap- 

 plicable to all the groups of strata in which traces of life occur. If 

 the calcareous rocks be included, which were of slower accumulation 

 and from different causes, it will not vitiate the process : limestones 

 being included in each great natural system of strata, our unit of 

 time may be supposed affected by equal errors in all the systems. 

 We may assume any convenient thickness, so as to include the prin- 

 cipal varieties of watery aggregates — as limestone, sandstone, clay 

 (reckoning conglomerates with the sandstones, shales with the clays, 

 and chalk with the limestones). Take, then, the unit of thickness 

 such that it shall be T^th of the ascertained strata in which life- 

 traces occur ; take the. thicknesses of the strata at their maximum 



