C. Lloyd Morgan — Geological Time. 161 



We must be careful, however, not to confuse the maximum thickness 

 given by Professor Ramsay with the average thickness of the strata. 

 In diffei'ent parts of England the same series of strata will often 

 vary much in thickness. For example, a band of Oolitic rocks runs 

 across England from the coast of Yorkshire to that of Dorsetshire. 

 Near Cheltenham the thickness is more than 500 feet, while in 

 Dorsetshire it is found to be reduced to less than 25 feet ! Other 

 instances might be given to any extent. The Kimmeridge Clay 

 near Oxford is some 90 or 100 feet thick ; but in the Sub-wealden 

 boring near Hastings it was found to have a thickness of some 700 

 feet. We may therefore fairly consider that the maximum estimate, 

 seventy-two thousand feet, is considerably in excess of the average 

 thickness of the sedimentary rocks of Great Britain. The argument, 

 too, as to the amount of rock swept away in past times by denuda- 

 tion, must not be pushed too far. In many parts, indeed, whole 

 groups of strata have disappeared from the rocky structure of our 

 island from this cause ; but in other parts these groups are well 

 represented. From the fact also that during the elevation of our 

 islands the strata must have been tilted so as to assume an inclined 

 position, denudation has exerted its destructive influence on the 

 edges of the strata, and therefore has not precluded us from estimat- 

 ing their original thickness. It is as if a fire had occurred in our 

 geological library, which did indeed destroy a great mass of 

 literature, but acting upon the exposed edges of the books, left a 

 portion of each in its place, which portion represents the thickness, 

 but not the size of the book. 



But there is another fact to be taken into consideration. Professor 

 J. Young (Pres. Address, Sect. C, Brit. Assoc, 1876) has pointed out 

 that our method of arranging the rocks in one long series, from 

 Laurentian to historical times, is erroneous, and leads us to give an 

 excessive estimate of the thickness of our strata. He argues that 

 some groups, which we are wont to arrange in a continuous vertical 

 series, were in reality contemporaneous, and should be arranged side 

 by side in parallel series. Our strata do not furnish us, as is 

 generally taught, with one continuous though mutilated volume, but 

 with two or three contemporaneous and also mutilated volumes, 

 bound up together. Let us consider for a moment the strata now in 

 process of formation in the neighbourhood of our islands. In the 

 English Channel sand is being deposited; further west, in the deeper 

 stiller water south of Ireland, fine mud is being laid down ; and 

 somewhat further west, in the deeper Atlantic, Glohigerina ooze is 

 being formed. Now it can hardly fail to happen that in many 

 places near the boundaries of these deposits fine mud will be laid 

 down on the top of Glohigerina ooze, on the one hand, and sand on 

 the fine mud, on the other hand. But how erroneous would be the 

 conclusions of the future geologist, who, arguing from the fact that 

 he found in one area the clay lying upon chalk, and in another sand 

 upon the clay, placed these strata in one continuous series, and took 

 the maximum thickness of each. In this way he might easily form 

 an estimate at least double the true thickness of the formations. 



DECADE II. VOL. T.— NO. IV. 11 



