262 SCIENCE PROGRESS 



in the Carboniferous. Carbon dioxide has a special solvent effect 

 on limestone, over and above all other kinds of rock, and so far 

 as this was exposed on hill-tops and in cliffs facing the sea, the 

 solvent effect might be much greater. Some small allowance 

 would probably be required for greater erosion in underground 

 caverns. But, to all this, there is a very definite limit. The 

 erosion could not, except under special conditions, affect the 

 limestone so as to take it below the level of the surrounding 

 country. If this happened, lakes would form, and the remaining 

 limestone would be covered with a protecting layer of shale. 

 The dependence of special erosion on general erosion is shown 

 by the fact that salt beds are so extensive and so numerous. 

 No possible conditions could make the solubility of limestone 

 approach that of salt in water. Yet salt beds are very slowly 

 removed to the sea, and it seldom, if ever, happens that we can 

 detect their presence by the greater salt content of river water. 

 With these remarks, the objection must be left. Like so much 

 other geological controversy, it appears to have been made 

 because of the supposed necessity to " hurry up " geologic 

 phenomena, so as to make them fit the dogmas of the physicist. 

 But the assumption of comparative uniformity is the soundest 

 that can be made. 



Without, however, dogmatising concerning details such as 

 these, we must note how important, in its relation to geologic 

 time, is the question of the evolution of carbonate of lime, both 

 in general and in special geological epochs. It is a consideration 

 on which considerable stress should be laid. 



Very brief mention must suffice for the one other method that 

 is now attracting attention. I refer to the estimation of the 

 amount of helium and of lead in minerals containing appreciable 

 quantities of uranium. The elements uranium and thorium, as 

 the modern chemist has abundantly shown, are slowly disinte- 

 grating and giving rise to other elemental forms. Assuming 

 that the helium found in these minerals is obtained from the 

 radioactive elements contained in them, an estimate of the time 

 that has elapsed since they were formed can be made. The 

 work of Mr. R. J. Strutt l has placed beyond doubt that, on that 

 assumption, the time that has elapsed since geologic epochs, not 

 the most ancient, must be measured in hundreds of millions of 

 years. But accurate and entirely self-consistent results have not 

 1 See various papers in the Proceedings pf the Royal Society. 



