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ME. PEESTWICH ON THE G-EOLOG-Y OF THE DEPOSITS 
Fig. 18. 
M. Original level of the country at the time of the formation of the high-level gravel d! d'. 
M'. Present level of the country, with the remaining portions of the high-level gravel d d. 
M M'. Supposed extent of elevation between the two periods. The dotted lines mark portions of the substrata 
successively raised to the level c 1 <? c 3 c 4 , and consecutively removed by the denuding action, the total amount 
of denudation being represented by C (or the space embraced between d and c 4 ). c 1 to c 4 may represent 
any thickness of strata ; the rate of elevation from M to M' may have been continuous or interrupted or 
partial, and the extent of elevation variable in different districts. According to any variability in the rate of 
elevation, to intervals of repose, or to deflections in the flow and velocity of the river, so there may exist 
intermediate terraces or levels, sudden variations in the slopes, and gravels lodged on different levels. As 
these not unfrequently occur, they often add much to the complexity of the problem. 
§ 7. THE QUESTION OF TIME AND SUCCESSION. 
In looking back at the subjects we have discussed, we are forcibly reminded of our 
dependence on the value of probabilities. On various points geology has not at present, 
and probably never will have, any other means of inference. All that can be done to 
give weight to our argument is to multiply probabilities, and by attending to the general 
concordance to reduce to the minimum the chances of error. The difficulty of one 
branch of the inquiry is considerably increased by the circumstance that the recent 
researches of naturalists tend for the present to give less security to any argument 
founded upon analogy of past with recent life. The case of the adaptation of the large 
extinct Pachyderms to a rigorous northern climate has long since deprived the remains 
of such genera of any weight with reference to the climatal conditions of past periods. 
It has now further become a question with some distinguished naturalists whether even 
the distribution of recent species is originally dependent on the influence of climate — 
whether the existence of certain kinds of food, the presence or absence of certain other 
animals, may not have been amongst the causes regulating the range of the animals. 
It is certain that the experience gained of late years of the facility of acclimatization 
indicates the necessity of caution. Nevertheless, in the absence at present of sufficient 
data with regard to this power of adaptation, we can only in the mean time rely on the 
evidence furnished by recent life so far as it regards species of known habits and range, 
provided especially it be supported by independent collateral proof. 
I will now proceed to make a few remarks on the question of time. We have to look 
at it both with reference to geological time, or the order of succession, which is merely 
relative, and to that which in the present instance more concerns this particular 
inquiry — the actual date of the existences and changes under consideration. With 
regard to geological time, I have before shown that the period is subsequent to that of 
the Boulder Clay — consequently to that of the great extension of the European glaciers 
