160 C. Lloyd Morgan — Geological Time. 



place, and the process may have set free a very considerable quan- 

 tity of heat ; in other words, when the energy which had before 

 been occupied in preventing combination became no longer equal 

 to the task, it appeared as sensible heat." That the data, on which 

 these speculations concerning solar radiation are conducted, are not 

 of a perfectly certain nature, may be gathered from the following 

 quotation from Professor Tait's sixth lecture : " The very lowest 

 estimate which we can make of the capacity of the sun for heat is 

 such that, cooling at the present rate — losing energy at its present 

 rate — the sun cannot possibly cool more than a single degree Centi- 

 grade in seven years. It may be, on the highest estimate we can 

 take, one degree in seven thousand years ; the data are very un- 

 certain ; but we may say that these are the limits between which it 

 must lie." 



Physically speaking, therefore, we may consider that geological 

 time must be comprised within limits of from ten to one hundred 

 millions of years. 



Let us now turn to the other side of the question, and consider 

 past time from the geological standpoint. The questions for con- 

 sideration are obviously these. In the first place, what is the 

 average thickness of the sedimentarj'- formations of some definite 

 area, say Great Britain ? in the second place, at what rates were they 

 severally and collectively deposited ? and lastly, what was the length 

 of time occupied in their deposition ? 



It is essentially necessary that we should confine our attention to 

 the thickness of the rocks of a definite area. We know next to 

 nothing of the geology of the world taken as a whole. Living as 

 we do on the land, we have only the power of studying at most 

 somewhat less than one-fourth of the earth's surface, and of that one- 

 fourth but a very small portion is actually known to us. Mr. Croll 

 has calculated that, assuming the duration of geological time to be 

 one hundred million years, at the present rate of denudation the 

 total mean thickness of rock formed during that time cannot exceed 

 five thousand feet. This may be so, and Mr. Croll's supposition, 

 that in the depths of the Atlantic, Pacific and Indian Oceans, "little 

 or no stratified deposits may exist," may possibly be true. Our 

 wisest course, however, in the existing state of knowledge, is, as it 

 seems to me, to study our own group of rocks, the conditions under 

 which those rocks were formed, and the evidence which we have, or 

 have not, for long breaks in the continuity of their deposition. This 

 is the task to which I will now apply myself. 



Professor Kamsay, Director-General of the Geological Survey of 

 Great Britain, tells us that the sedimentary formations of that area 

 have a maximum thickness of upwards of seventy-two thousand feet, 

 (thirteen and a half miles). But to this must be added, say some 

 geologists, "the quantity of rock removed during past ages by 

 denudation." " In many places," writes Mr. Croll, " the missing 

 beds must have been of enormous thickness. The time represented 

 by beds which have disappeared is, doubtless, as already remarked, 

 much greater than that represented by the beds which now remain." 



