l68 GEOLOGY [CHAP. IX 



Letter 514 later changes of the earth's surface. Reading your book has 

 brought vividly before my mind the state of knowledge, or 

 rather ignorance, half a century ago, when all superficial 

 matter was classed as diluvium, and not considered worthy 

 of the attention of a geologist. If you can spare the time 

 (though I ask out of mere idle curiosity) I should like to hear 

 what you think of Mr. Mackintosh's paper, illustrated by a 

 little map with lines showing the courses or sources of the 

 erratic boulders over the midland counties of England. 1 It 

 is a little suspicious their ending rather abruptly near Wolver- 

 hampton, yet I must think that they were transported by 

 floating ice. Fifty years ago I knew Shropshire well, and 

 cannot remember anything like till, but abundance of gravel 

 and sand beds, with recent marine shells. A great boulder 2 

 which I had undermined on the summit of Ashley Heath, 

 720 (?) ft. above the sea, rested on clean blocks of the under- 

 lying red sandstone. I was also greatly interested by your 

 long discussion on the Loss ; 3 but I do not feel satisfied that 

 all has been made out about it. I saw much brick-earth near 



1 " Results of a Systematic Survey, in 1878, of the Directions and Limits 

 of Dispersion, Mode of Occurrence, and Relation to Drift-Deposits of the 

 Erratic Blocks or Boulders of the West of England and East of Wales, 

 including a Revision of Many Years' Previous Observations," D. Mack- 

 intosh, Quart. Journ. Geol. Soc., Vol. XXXV., p. 425, 1879. 



2 Mackintosh alludes (loc. tit., p. 442) to felstone boulders around 

 Ashley Heath, the highest ground between the Pennine and Welsh hills 

 north of the Wrekin ; also to a boulder on the summit of the eminence 

 (774 ft. above sea-level), " probably the same as that noticed many years 

 ago by Mr. Darwin." In a later paper, " On the Correlation of the Drift- 

 Deposits of the North-West of England with those of the Midland and 

 Eastern Counties" (Quart. Journ. Geol. Soc., Vol. XXXVI., p. 178, 1880) 

 Mackintosh mentions a letter received from Darwin, " who was the first 

 to elucidate the boulder-transporting agency of floating ice," containing 

 an account of the great Ashley Heath boulder, which he was the first to 

 discover and expose, ... so as to find that the block rested on frag- 

 ments of New Red Sandstone, one of which was split into two and deeply 

 scored. . . . The facts mentioned in the letter from Mr. Darwin would 

 seem to show that the boulder must have fallen through water from floating 

 ice with a force sufficient to split the underlying lump of sandstone, but 

 not sufficient to crush it." 



3 For an account of the Loss of German geologists" a fine-grained, 

 more or less homogeneous, consistent, non-plastic loam, consisting of an 

 intimate admixture of clay and carbonate of lime," see J. Geikie, loc. 



p. 144 et seq. 



