Let me briefly recall the "Age of the Earth controversy". Based on the hypothesis that Earth was originally hot and cools down ever since, and using the data on the gradient of temperature currently observed near the surface (about 1 degree per 100 meters), Willian Thomson (Lord Kelvin) derived that it must take about 100 million years to cool to the present state, starting from the melting temperature of rocks). Later he revised his estimate to a smaller number.
This theory, though completely mathematically sound gives the wrong answer because it does not take into account the heat source inside (radioactivity, which was not known at that time). Mathematics of Kelvin's theory is completely clear. But my question is different:
I know from many popular science histories that Kelvin's estimate was hotly disputed by geologists and evolutionists, who all thought that Earth must be much older "to allow sufficient time" for geological changes and for evolution.
My question is:
How exactly did they estimate the time necessary for geological changes and evolution? How can you arrive with ANY numerical estimate of this sort?
For example Darwin himself presumably had estimated the time necessary for evolution as 300 million years. But he removed this computation from the third edition of his book (Origin of species). As I only can find later editions, I cannot see what arguments he used to obtain his estimate.
There were many other such estimates, for example, Samuel Haughton, a geologist, came with the lower estimate of 2300 million years. What sort of arguments could all these scientists (I mean geologists and evolutionists) use to obtain their numerical estimates from below.
Added later: I sort of understand the arguments of geologists referred in the comments. But how 19 century evolutionists estimated the rate of evolution is still completely unclear to me.