1868.] MACKINTOSH SUlll ACES OK LIMESTONE AND GRANITE, 277 



at any part of the earth's surface, we might even find indications of 

 tropical and of polar climates alternating with each other in any 

 order of succession. But the theory of the continual change of 

 isothermal surfaces is not confined to the narrow limits of the planet 

 we inhabit. Wherever the causes I have referred to are in action, 

 similar changes must be taking place. Our own satellite as well as 

 all those attached to planets in our own or in other systems obey 

 the same law. There is sufiicient evidence that the amount of the 

 radiation of heat from the sun itself is continually changing. This 

 change Avill necessarily cause analogous changes in all its dependent 

 planets. The stars themselves in many cases have given indication 

 of change in the intensity of their light. In one or two instances 

 a star has almost suddenly become brilliant, and after a short 

 period nearly resumed its former appearance. 



I have entered thus fully into the history of the changes of iso- 

 thermal surfaces within the earth, because, during a late visit to Cam- 

 bridge, my valued friend Professor Sedgwick informed me that 

 he always concluded his annual course of geological lectures by ex- 

 plaining " HerscheVs theory of the change of the isothermal surfaces 

 within the earthP I referred him to the dates, and informed him 

 that, whilst I fully admitted the originality of my friend's discovery, 

 I claimed for myself its long previous discovery and publication. I 

 delayed the printing of a treatise then going through the press. I 

 caused woodcuts to be made to illustrate Sir John Herschel's letter 

 to Sir Charles Lyell, and was the first to publish my friend's subse- 

 quent, although qidte independent, discovery to the world, in the 

 Notes to the first edition of the Ninth Bridgwater Treatise. The 

 date of the Preface to that work is April 1837. 



2. Gn the Origin of Smoothed, Rounded, and Hollowed Surfaces 

 of Limestone and Granite. By D. Mackintosh, Esq., F.G.S. 

 (Abstract.) 

 The author, believing that late, discussions on denudation have 

 reached a stage which requires that the respective claims of rain- 

 water and sea-waves to mould rock-surfaces should, as far as 

 possible be set at rest, proceeds to state the results of his observa- 

 tions as follows : — 



'' Unless rain-water be retained for a certain length of time in 

 previously formed hollows, it can exert little or no chemical infiuence 

 on rock-surfaces ; and therefore the principal, if not all, the in- 

 equalities found on the sloping or vertical sides of fragments, blocks, 

 or faces of rock must be either the original fractured surfaces, or pro- 

 duced by some kind of mechanical action. Eain-water, uncharged 

 with solid matter, such as sand, can exert little or no mechanical 

 power on hard rocks. The chemical action of rain-water can en- 

 large previously formed hollows ; and where retained for a sufficient 

 time, it can dissolve out fossils, and develope the structural form of 

 rocks. Rain-water, not concentrated into streams of sufficient force, 



