38 ME, A. M. DAVIES OX THE KIMERIEGE CLAY [Feb. 1907, 



calcareous nor purely argillaceous, although it adheres to the tougue. 

 In one or two cases, small patches of actual grey clay replace the 

 ordinary matrix, and in them the ellipsoids are scattered more 

 loosely. The rock, as a "whole, is very light and porous, absorbing 

 water very readily. These characters, taken together, enable 

 fragments of the rock to be identified with the greatest ease. 



In one place only — at a pond-digging at the "Warren Farm, north 

 of Studley l — this rock undergoes a remarkable change, becoming a 

 limestone, though still exhibiting the characteristic structure. If we 

 treat a piece of this limestone with acid, the great majority of the 

 ellipsoidal grains dissolve, but a few of them, along with some 

 argillaceous matter, remain as an insoluble residue. 



If some of the grains are removed from the ordinary Arngrove 

 Stone and examined under the microscope, they are found to agree 

 exactly in size and shape with the globate spicules of the tetractinellid 

 sponge Rhaxella perforata, described by Dr. G. J. Hinde 2 from 

 cherts in the Lower Calcareous Grit of Yorkshire. They are about 

 •15 mm. in length, -1 mm. in sectional diameter, and occasionally 

 show a distinct hilum. When they are mounted in glycerine (but not 

 when in balsam) the regular quincuncial surface-marking can be 

 detected under a high power, although in the blue spicules it is 

 obscured by a coarser network-structure. Another, more striking 

 difference between the blue and white spicules is, that between 

 crossed nicols the former show first-order yellow interference- 

 colours and an excentrically-radial structure, while the latter are 

 practically isotropic. 



The examination of a thin slice shows that, in addition to the two 

 kinds (or conditions) of spicules, there are also a few angular and 

 subangular grains of quartz of the same order of magnitude. The 

 proportion of ' blue ' spicules to ' white ' spicules to quartz-grains 

 averages 9 to 3 to 1 in one slide. 



The ' blue ' spicules, when cut tangentially, show the irregular 

 network-structure mentioned above. When cut centrally, they 

 show (1) a narrow outermost zone, nearly clear and almost isotropic ; 

 (2) within this, a dusky-brown zone, equally narrow, its inner 

 margin showing cuspation, probably corresponding to the network 

 apparent on the surface ; (3) a clear, glassy centre, forming the 

 main bulk of the spicule. This last, on careful examination, often 

 shows a faintly-marked agate-like structure ; and, between crossed 

 nicols, a corresponding radiating chalcedonic structure is revealed. 

 The radiation is sometimes fairly regular about the centre of the 

 spicule ; at other times it is of a more complex type, with several 

 centres ; or, aaain, the radii mav start from a point on the margin. 

 (See PL I, fig. 2.) 



1 This must not be confused with the (geologically more famous) Warren 

 Farm near Stewkley, 17 miles to the east-north-east. Another Warren Farm, 

 near Holton, -will be mentioned farther on (p. 4*J). 



2 ' British Fossil Sponges' Monogr. Paheont. Soc. pt. iii (1893) pp. 210-12 & 

 pi. xiii, figs. 7, 7 a-f; and Quart. Journ. Geol. Soc. vol. xlvi (1890) pp, 54-61 

 & pi. Yi. 



