1878.] GEOLOGY. 235 



established, and then taking the graduated changes of the , 

 same group of organisms as the true standard ! I never 

 hoped to live to see such a step even proposed by any one." 



Another geological research which roused my father's 

 admiration was Mr. D. Mackintosh's work on erratic blocks. 

 Apart from its intrinsic merit the work keenly excited his 

 sympathy from the conditions under which it was executed, 

 Mr. Mackintosh being compelled to give nearly his whole 

 time to tuition. The following passage is from a letter to 

 Mr. Mackintosh of October 9, 1879, and refers to his paper in 

 the Journal of the Geological Society, 1878 : 



"I hope that you will allow me to have the pleasure of 

 thanking you for the very great pleasure which I have derived 

 from just reading your paper on erratic blocks. The map 

 is wonderful, and what labour each of those lines shows ! I 

 have thought for some years that the agency of floating ice, 

 which nearly half a century ago 'was overrated, has of late 

 been underrated. You are the sole man who has ever noticed 

 the distinction suggested by me * between flat or planed 

 scored rocks, and mammillated scored rocks."] 



C. Darwin to C. Ridley. 



Down, November 28, 1878. 



DEAR SIR, I just skimmed through Dr. Pusey's sermon, 

 as published in the Gtiardian, but it did [not] seem to me 

 worthy of any attention, As I have never answered criticisms 

 excepting those made by scientific men, I am not willing that 

 this letter should be published ; but I have no objection to 

 your saying that you sent me the three questions, and that 

 I answered that Dr. Pusey was mistaken in imagining that I 

 wrote the ' Origin ' with any relation whatever to Theology. I 

 should have thought that this would have been evident to 



* In his paper on the ' Ancient Glaciers of Carnarvonshire,' PhiL 

 Mag. xxi. 1842. See p. 187. 



