14 Plant-life of the Oxford District 



a time-period of some 150 millions of years ago, there is ample time for 

 a quite indefinite succession of such horizons, as depression to varying 

 depths, and subsequent elevation and denudation, may involve successional 

 strata of calcareous coralline deposits, clays and shales, sandstones and 

 limestones, as the expression of rhythmic oscillations of even million-year 

 periodicity. Successional strata of clays and calcareous sands and limestones 

 in alternating series are the characteristic feature of the district, and such 

 successive strata do not lie conformably on one another ; as these main 

 earth-movements have been associated with minor local oscillations, the 

 effect of volcanic disturbances, and the possibility of repeated exposure as 

 a land-surface, subject to subaerial denudation which may cut through several 

 older beds. 



Such periods may have been repeated over and over again for indefinite 

 hundreds of millions of years before. Rocks of Liassic age thin out below 

 the district, implying that there was more land exposed at that time ; and 

 these in turn rest unconformably on the far older deposits of the Carbonifer- 

 ous, below which are indications of deposits of Silurian seas. Taking the 

 Carboniferous as some 400 million years ago, and the Silurian as 600, even 

 these horizons are probably but a short way to the time of the first elevation 

 of land with the sea-weed vegetation of the sea. 



The entire history implies indefinitely continued oscillatory exchanges 

 of an old continental seaboard, with occasional variants, as larger denuda- 

 tions introduce estuarine conditions at the outflow of great continental rivers. 



Omitting the higher beds of sands (Greensand) and clay (Gault) which 

 are left as mere relics on the tops of surrounding hills, the Oxford district is 

 built on three main systems : 



(i) Oxford Clay, giving clay soils at lower levels. 



(a) Calcareous Grit and Coral Rag (Corallian Series), on rising ground. 



(3) Kiineridge Clay, giving clay soils at higher elevations, and also on 

 low ground to the SE. 



In further detail, the Oxford Clay, though normally following an 

 indefinite sequence of marine deposits of Coralline limestone of the Lower 

 Oolite (Jurassic), to the Lias touched at a depth of 430 ft. below the 

 City, as : 



(A) Inferior Oolile (Limestone), Chipping Norton. 



Great Oolite (White Limestone, ' oolite ', and marl) with estuarine beds. 

 Forest Oolite (Clay and Limestone), Hanborough, Islip, Witney. 

 Cornbrash (Limestone), Witney Bridge, 

 is in turn covered locally by : 



(B) Lower Corallian of Calcareous Grit and Sands, Headington, Cowley, Littlemore. 

 Upper Corallian of Coral Rag and Coralline Oolite, Headington Quarry, 



Cumnor Hill. 



A deposit of a foot of silt in 1,000 years while moving would give 300 ft. of deposit in 300,000 ; and 

 at the same rate of re-elevation 200 ft. to the sea-surface, a million years becomes a small unit in 

 accounting for the depression, deposition, and elevation of such a geological horizon. On a globe 

 i metre diameter the total range of such oscillation would be represented by a distance of 10 \i, 



These speculations are warranted from the estimates given by chemists for the disintegration of 

 radio-active minerals found in older rocks. Such data are accepted, in absence of any other means 

 of calculation, because they suggest agreement with the enormous distances of time required to 

 explain the slow evolution of plant-life, as itself the most delicate response to the changing conditions 

 of geological time (Holmes, 1920, Discovery, p. 113). 



It is hardly realized yet that the enormous times suggested by these chemical deductions supply 

 the one factor needed to make the biological machine work throughout the ages, at a rate no 

 greater than things are changing at the present time ; and that it was for want of these remarkable 

 data that previous generations of biologists have failed to grasp the full significance of ' natural 

 selection ' over spaces of time, which to our senses are inconceivable, and what the history of life on 

 this world really implies. At the Brit. Assoc. Meeting, 1921, Lord Rayleigh suggested some 8,000 

 million years as the possible age of the earth, and geological ' time ' begins to bear a reasonable 

 relation to astronomical ' space '. 



