THE BAGSHOT BEDS OF THE LONDON BASIN. 



381 



.-ja much greater areal extension originally of the Upper Sands, 

 which harmonizes with the view advocated in this and in former 

 papers. 



9. Lastly, the general homogeneity of the Upper Sands, when 

 viewed on a large scale, as contrasted with the numerous lithological 

 variations, laterally as well as vertically, of the Middle and Lower 

 beds, points to such an agency as a tidal surf, in workingup the various 

 materials, brought into the estuary from different parts of the catch- 

 ment-basin, into a somewhat homogeneous whole, as we see them in 

 the Upper Sands ; while the absence of such an agency in the Lower 

 and Middle stages led to that differentiation of the deposits just 

 referred to, the materials contributed by each affluent being more or 

 less locally deposited. These lateral variations in thickness and 

 lithological character of beds on the same horizon constitute, indeed, 

 the chief difficulty of their stratigraphy, and discount largely the 

 value of conclusions drawn from a comparison of small and distant 

 sections. Such evidence standing alone cannot be other than 

 equivocal. 



Freshwater Diatoms Sfc. — It would be premature to say much 

 of these until the investigation has been carried further. Up 

 to the present they have only been found in any quantity in three 

 beds in different localities, though many samples have been searched 

 and stray forms have been found in others. The sands examined 

 have been washed with distilled water. Their fossil condition seems 

 to be indicated (l)by the absence of endochrome, (2) by the corroded 

 and fragmentary condition of many of them *. The work of Prof. 

 E. PfTtzer, of Heidelberg, has been mainly consulted in this study. 

 He confirms the previous judgment of Smith as to the distinction 

 between marine and freshwater forms, and points out their import- 

 ance from this point of view as geological data. He assigns ^ 

 millim. as the limit of size of freshwater species ; those which have 

 been found in the Bagshot Beds seldom reach | miliim. in length. 

 " Along with empty frustules of Diatoms," he says (loc. cit. p. 408), 

 " only spicules of freshwater sponges and silicified remains of many 

 higher plants are generally present." The latter, in the form of 

 vascular tissue, appear to be more common in the Bagshot Beds than 

 Diatoms. They are distinctly luminous in a dark field between 

 crossed nicols, and remain unchanged after being calcined for more 

 than a quarter of an hour on clean platinum foil. I am not sure 

 that I have seen any sponge-spicules. The following may be men- 

 tioned as genera that have been met with: — Gomphonema, Frustidia, 

 Pinnularia, Synedra, Gyrosigma, Navicula, Odontidium, Melosira, 

 Amphipleura. They occur both in the Middle and Lower Bagshot, 

 and none have been met with in the Upper Bagshot. [In a sample 

 of the Kieselguhr deposit lately discovered at L. Quire in Skye, for 

 which I am indebted to the courtesy of A. Macdonald, Esq., of 



* Vide Schenk's ' Handbuch der Botanik ' (Breslau, 1882), Bd. ii. pp. 409 et 

 seq., also fig. 1 of that monograph (p. 407). I offer here my best thanks to 

 Prof. Marshall Ward, of Cooper's Hill College, for directing my attention to 

 this, Pfitzer's latest published work. 



Q. J. Gr. S. No. 171. 2d 



