524 Correspondence — Mr. 0. Fisher. 



At tlie close of the Bunter period, elevation took place, in the 

 Midlands certainly, if not generally throughout the country, accom- 

 panied by extensive and long-continued denudation : during this 

 interval of time the land appears to have been cut up by subaerial 

 erosion, and its surface furrowed, by channels. 



On depression again setting in at the commencement of the Keuper 

 period, these hollows appear to have been first filled up by coarse 

 sediments that were drifted along by powerful westerly currents. 

 This will suffice to explain the local development and rapid fluctua- 

 tions in thickness of the Keuper Basement Beds. 



In lithological chai-acter the Keuper Basement Beds show marked 

 differences both from the underlying Bunter Sandstones and the 

 overlj'ing Waterstones ; though there are beds in the series which 

 strongly resemble one or other of those rocks. Typicalhj, however, 

 the texture of the Basement Beds is essentially Keuper-like : the 

 grains of sand, whether coarse as any Bunter or fine as any Keuper 

 sandstone, are mostly angular, clean, and of a flat and elongated or 

 schistose type, while those of the Bunter are of a more globular or 

 granitic type, and generally stained with a coating of ferric oxide ; 

 mica too is much more abundant than in most Bunter Sandstones. 

 The quartzites of the Keuper Basement Beds count for less than 

 nothing, as they have all the appearance of having been derived 

 from the waste of the Bunter Pebble Beds. 



I quite agree with Mr. Strahan that the Keuper Basement Beds, 

 as defined by him, form a distinct subdivision of the Triassic series, 

 and that whilst the Waterstones graduate up into the Eed Marls, and 

 their division therefrom is arbitrary, the Basement Beds are sharply 

 separated from the rest of the Keuper. 



In conclusion, I would recommend the retention of the term 

 " Basement Beds " applied to these rocks as more expressive and 

 less open to misinterpretation than that liorribly confusing phrase 

 " Lower Keuper Sandstone." E. Wilson. 



^Nottingham, 5 Oct. 1881. 



ME. FISHEE'S EEPLY TO ME. DAY'S CEITICISM. 



SiK, — If Mr. Day will consider what I meant by " obliquity of 

 trend," in the trace of a furrow, made by a railway cutting, he will 

 perceive that my assertion is correct, that " no apparent obliquity 

 of trend can be given by a vertical section, e.g. by a vertical cliff'." 

 I believe that the difference between us arises from our understand- 

 ing the obliquity to apply to different angles.' 



To use Mr. Day's illustration of a shadow : let the shadow, made 

 by horizontal rays, of a vertical square, whose sides are vertical and 

 horizontal, fall upon a plane which is not parallel to the plane of 

 the square. So long as the plane which receives the shadow is 

 vertical (which answers to the vertical cliff), the shadow of a vertical 

 edge of the square will be perpendicular to a horizontal line drawn 

 on the plane. But if the plane be inclined to the horizon, the shadow 

 of the vertical edge of the square will no longer be perpendicular to 

 the horizontal line. It is the angle between the shadow of the edge 

 1 See Mr. Fisher's article, January, 1881, p. 20. 



