WALTOX-COUMOX SECIIOX. 



449 



6. Peobable Geological Position oe the Hatch Bbjck-eakth. 



In attempting to fix the position of this remarkable loam, or brick- 

 earth, we must first arrive at an understanding of what is meant by 

 Middle Bagshots. A casuist might argue, because Prof. Prestwich 

 speaks of the clays between Addlestone and Chertsey as forming the 

 base of the Middle Bagshots, that in point of fact these must belong 

 to the Middle Bagshots, whatever the position of other beds assigned 

 to that series may be. As a mere logical crux, there may be something 

 to be said for this view of the case ; but geologists will have no 

 difficulty in admitting, if the basal beds of the Middle Bagshots can 

 be shown to occur on a higher horizon in this district, and if such 

 beds have been recognized, both by Prof. Prestwich and the Officers 

 of the Geological Survey, as forming the base of the Middle Bagshots, 

 that in that case we must take the beds usually accepted as the base 

 of the Middle Bagshots for our standpoint, and then see if the Hatch 

 brick-earth or loam can be brought into alignment with them. 



Along the line of the London and South- Western Railway it is 

 clear, according to the meaning both of Prestwich and the Survey, 

 that the brick- earth (loam) worked on St. George's Hill forms part of 

 the basal beds of the Middle Bagshots — there 170 ft. above O.D. 

 But the type section must be sought in the cutting on Goldsworthy 

 Hill, where Prof. Prestwich describes the basal beds of the Middle 

 Bagshots as foliated clays, more or less sandy, having a thickness of 

 14 feet and resting on 130 feet of "Lower Bagshot Sands." 



We are not now discussing the question as to what are the best 

 divisions for the Bagshot series of the London basin taken as a whole. 

 The real issue to be decided at present is whether the Hatch brick- 

 field is in alignment with the basal beds of the Middle Bagshots as 

 defined by Prestwich in the Goldsworthy cutting ; if it is below that 

 horizon it should be mapped as part of the Lower Bagshots notwith- 

 standing its argillaceous character *. I am disposed to think that 

 it does lie below the basal beds thus defined, and moreover that it 

 differs somewhat in character from the basal beds of the Middle 

 Bagshots as seen in the clay-pit on St. George's Hill. 



The considerations for determining the point at issue are partly 

 stratigraphical and partly lithological. We must not, I admit, place 

 too much reliance on the latter, considering the variable and uncertain 

 nature of such accumulations. I would merely indicate that the 

 general character of the standard basal clays of the Middle Bagshots 

 is much more regular, there is less of such very accentuated false- 

 bedding, and the material is more frequently of the nature of a pipe- 

 clay. At the same time there are certain well-known features 

 common to all Bagshot clays, such as their loamy and laminated 

 character, abundance of carbonaceous matter, and other conditions, 

 all pointing to considerable similarity in origin. Hence we must 



* It may perhaps be a legitimate question how far physical peculiarities should 

 determine the mapping of a series. If by Lower Bagshots it is intended to 

 represent sandy beds, and by Middle Bagshots loamy or clayey beds, then 

 Woburn Hill is correctly mapped. But although this arrangement is suitable 

 to the economy of the case it cannot be satisfactory to geologists. 



