448 A. J. Jukes-Broivne — Boring at Marston, Devizes. 



Cardmm striatulum, Sow. "With the sample was a piece of flint, and 

 the foreman in charge reported that the tool had passed through 

 a layer of stone. If the flint was supposed to be a sample of the 

 layer of stone, it is obvious that there was no such layer and that the 

 flint had fallen down the bore from the surface. This illustrates how 

 mistaken records of stone layers may get into contractors' accounts 

 of borings. 



A sample from 314 feet is a stiff grey clay with little or no sand 

 and does not contain any recognizable fossils. The next sample was 

 from 355 feet and is a somewhat shaly clay of a medium-grey tint ; 

 it contains a few fragments of small shells. 



No more samples were sent till a depth of 400 feet was reached, 

 the statement tlien being that the clay had continued to be similar to 

 that sent from 355 feet, but the sample from 400 feet was different, 

 being a compact homogeneous dark-grey clay, cutting like cheese, as 

 if there was much organic matter in the material. At 412 feet the 

 clay was again different, being lighter in colour, not of a cheesy 

 texture, but full of small shell-fragments. 



From the facts above recorded I think we may conclude that the 

 upper (Virgulian) division extends to at least a depth of 304 feet, and 

 as about 34 feet must be added at the top the total thickness of this 

 division must be at least 338 feet, and probably a few feet more. 

 The clays at and below 350 feet in the boring probably belong to the 

 Pterocerian or Lower Kimeridge, and the question is how much more 

 of the lower division is likely to exist in this part of AViltshire. 

 I can only see one way of attempting to solve this question, and that 

 is to assume that the proportional thicknesses of the upper and lower 

 divisions are the same as those of the two divisions in Dorset. In 

 1880 Blake published a diagram drawn, to scale of the relative 

 thicknesses of the several parts of the Kimeridge Clay at Kimeridge.^ 

 This showed a total of 1,000 feet, and assigned a thickness of about 

 420 feet to the lower division. Strahan, however, in discussing the 

 thickness of the Upper Kimeridge,' took the boundary-line between 

 it and the Portland Sands 100 feet higher than Blake did, which 

 makes the total 1,100 feet and the thickness of the Virgulian about 

 680 feet. Hence in Dorset the latter forms nearly three-fifths of 

 the whole stage. If, therefore, the propoi'tions are the same in 

 South Wilts, and if we assume the Virgulian to be 340 feet thick at 

 Marston, then the thickness of the Pterocerian will be 205 feet and 

 the total 545 feet. 



The importance of this boring is that it has proved the thickness 

 of the Kimeridge Clay near Devizes to be more than 446 feet, and has 

 afforded a basis for making a rough calculation of the probable total 

 thickness of the Clay in that district. It is a great pity that no fund 

 exists from which assistance could be obtained for continuing trial 

 borings of this kind, so that some definite result should be obtained, 

 and the expense incurred by the owner of the property should not be 

 entirely wasted. 



^ Quart. Jom-n. Geol. Soc, vol. xxxvi, p. 197, 1880. 



" Geology of the Isle of Purbeck, etc. (Mem. Geol. Surv.), 1898, p. 52. 



