EVIDENCE IN SUPPORT OF THE THEORY FROM THE AGE OF THE SUN’S HEAT.
Testimony of Geology and Biology as to the Age of the Sun’s Heat.
The question which we have now to consider is—to which of the two theories does geology lend its testimony? Will the length of time which, according to the gravitation theory, can possibly be allotted satisfy the requirements of geology? In short, are the facts of geology reconcilable with the theory? If not, the theory must be abandoned.
Before the period when geologists felt that they were limited to time by physical considerations, the most extravagant opinions prevailed in regard to the length of geological epochs. So long as the physicist continued to state in a loose and general way that the sun might have been supplying our earth with heat at his present rate for the past 100,000,000 years, no very serious difficulty was felt; but when geologists came to understand that ten or twenty millions of years were all that could be granted to them, the condition of matters was entirely altered. The belief that the mathematical physicist must be right in his views as to the age of the sun’s heat, and that there is no possibility of a longer period being admitted, seems at present to be leading geologists towards the opposite extreme in regard to the length of geological time. Attempts have been recently made to compress the geological history of our globe into the narrow space allotted by the physicist. The attempt is hopeless, as well as injurious to geological science. What misleads is not the belief that gravitation could not possibly afford a supply of heat sufficient for more than 20,000,000 years, for this is true; it is the belief that there was no other source of heat than gravity.
We shall now consider the evidence which geology seems to afford as to the age of the sun’s heat. Geology is quite competent to render aid on this point, for the sun’s heat must be at least as old as life on this globe; and the record of the rocks tells us when this life first appeared. We require, however, to be able to measure the time which has elapsed since these records were left. What we want is absolute time; not relative time. Much has been done by geologists in regard to relative time; but this can be of no service to us in our present inquiry. Unfortunately very little trustworthy work has been done in the way of determining the absolute length of geological periods. Happily, however, great exactness of measure is not required. A rough approximation to the truth will suffice for our present purpose. If it can be shown to be more than fifteen or twenty millions of years since life first appeared on the earth, it will as effectually prove that gravitation alone could not have been the source from which the sun derived his heat as if it were shown that that period was a thousand times more remote. All we have to do is simply to assign an inferior limit to the age of life on the earth; and this can be effectually done by means of the methods, imperfect though they be, which we have at command. As the question of geological time is of some importance in relation to our present inquiry, I shall consider it at some length.
Testimony of Geology: method employed.—What has subsequently proved to be a pretty successful method of measuring geological time suggested itself to my mind during the summer of 1865. It then occurred to me that we might obtain a tolerably accurate measure of absolute geological time from the present rate of subaërial denudation, which might be ascertained in the following way: The rate of subaërial denudation must be equal to the rate at which materials are carried off the land into the sea; and this is measured by the rate at which sediment is carried down by our river systems. Consequently, in order to determine the present rate of subaërial denudation, we have only to ascertain the quantity of sediment annually carried down by the river systems. This gives us the time required to remove any given quantity, say one foot, off the face of the country. If we assume the rate to be pretty much the same during past geological ages, we have a means of telling the time that was occupied in removing any known thickness of strata. But as we never can be perfectly certain that the rate is the same in both cases, the results can, of course, be regarded as only approximately true.
Taking the quantity of sediment discharged into the sea annually by the Mississippi river, as determined by Messrs. Brown and Dickson,[[23]] I found that it amounted to one foot off the face of the country in 1,388 years, and that, at this rate of denudation, our continents, even if they had an elevation of 1,000 feet, would not remain above sea-level over 1,500,000 years.[[24]] This was an exaggerated estimate of the quantity of sediment, for I shortly afterwards found that far more accurate determinations were made by Messrs. Humphreys and Abbot,[[25]] who were employed by the United States Government to report upon the physics and hydraulics of the Mississippi. Messrs. Brown and Dickson had estimated the quantity of sediment at 28,188,083,892 cubic feet, whereas Messrs. Humphreys and Abbot found it to be only 6,724,000,000 cubic feet, or less than one-fourth that amount. This gives one foot in 6,000 years as the rate of denudation.
At this time Dr. Archibald Geikie took up the question and went into the consideration of the subject in a most thorough manner; and it is mainly through the instrumentality of his writings on the matter[[26]] that the method under consideration has gained such wide-spread acceptance among geologists. After an examination of nearly all that is known regarding the amount of sediment carried down by rivers, he drew up the following table, showing the number of years required by seven rivers to remove one foot of rock from the general surface of their basins.
| Danube | 6,846 | years | |
| Mississippi | 6,000 | „ | |
| Nith | 4,723 | „ | |
| Ganges | 2,358 | „ | |
| Rhone | 1,528 | „ | |
| Hoang-Ho | 1,464 | „ | |
| Po | 729 | „ | |
| ----- | ----- | ||
| Mean | 3,378 | years |
This gives a mean of 3,378 years to remove one foot, or a little over one-half the time taken by the Mississippi. This mean appears to be generally taken as representing the average rate of subaërial denudation of the whole earth, but it has, I fear, been rather too hastily adopted. To estimate correctly the quantity of sediment annually discharged by a large river is a most difficult and laborious undertaking. A perusal of the voluminous report of Messrs. Humphreys and Abbot, extending over 690 pages, which Dr. Geikie justly styles a model of patient and exhaustive research, will clearly show this, and at the same time prove how skilfully and accurately the task allotted to them was performed.