l66 GEOLOGY [Chap. IX 



Letter 512 depth during a large part of the year with drifted frozen 

 snow, over which rubbish from the upper parts of the plat- 

 forms was washed by the summer rains, sometimes along one 

 line and sometimes along another, or in channels cut through 

 the snow all along the main course of the broad valleys. 



I suppose that I formerly mentioned to you the frequent 

 upright position of elongated flints in the red clayey residue 

 over the chalk, which residue gradually subsides into the 

 troughs and pipes corroded in the solid chalk. This letter 

 is very untidy, but I am tired. 



P.S. Several palaeolithic celts have recently been found 

 in the great angular gravel-bed near Southampton in several 

 places. 



Letter 513 To D. Mackintosh. 1 



Down, Nov. 13th, 1880. 



Your discovery is a very interesting one, and I congratu- 

 late you on it. 2 I failed to find shells on Moel Tryfan, 

 but was interested by finding (P/ii/osop/i. Mag., 3rd series, 

 Vol. XXL, p. 184) shattered rocks 3 and far-distant rounded 



1 Daniel Mackintosh (1815-91) was well known in the South of 

 England as a lecturer on scientific subjects. He contributed several 

 papers to the Geological Society on Surface Sculpture, Denudation, 

 Drift Deposits, etc. In 1869 he published a work On the Scenery of 

 Englci7id and Wales (see Geo/. Mag., 1891, p. 432). 



2 " On the Precise Mode of Accumulation and Derivation of the 

 Moel-Tryfan Shelly Deposits ; on the Discovery of Similar High-level 

 Deposits along the Eastern Slopes of the Welsh Mountains ; and on the 

 Existence of Drift-Zones, showing probable Variations in the Rate of 

 Submergence." By D. Mackintosh, Quart. Journ. Geo/. Soc, Vol. 

 XXXVII., pp. 351-69, 1 88 1. [Read April 27th, 1881.] 



3 In reviewing the work by previous writers on the Moel-Tryfan 

 deposits, Mackintosh refers to Darwin's " very suggestive description of 

 the Moel-Tryfan deposits. . . . Under the drift he saw that the surface 

 of the slate, to a depth of several feet, had been shattered and contorted in 

 a very peculiar manner." The contortion of the slate, which Mackintosh 

 regarded as "the most interesting of the Moel-Tryfan phenomena," had 

 not previously been regarded as " sufficiently striking to arrest attention " 

 by any geologist except Darwin. The Pleistocene gravel and sand 

 containing marine shells on Moel-Tryfan, about five miles south east 

 of Caernarvon, have been the subject of considerable controversy. By 

 some geologists the drift deposits have been regarded as evidence of a 

 great submergence in post-Pliocene times, while others have explained 



