Tol. 69.] TWO BEEP BOEINGS AT CALVEET STATION. 325 



;are: (1) Merevale (Warwickshire), over 50 miles north by west 

 from Calvert ; (2) Bronsil (Herefordshire), 60 miles west by north ; 

 and (3) Shineton (Shropshire), over 80 miles away to the north- 

 west. The new discovery, therefore, extends very considerably 

 the area over which Tremadoc Beds can be affirmed to have been 

 originally deposited, as there is no reason to doubt that these four 

 widely-separated occurrences were originally continuous. 



"We have no intention of entering into the discussion whether 

 the Tremadoc Beds are more naturally grouped with the Cambrian 

 or with the Ordovician. If, in the course of this paper, we refer 

 to them as Cambrian, it is solely for convenience : for, in their 

 geographical distribution in Britain, the Tremadoc Beds accompany 

 the Cambrian, and not the Ordovician. 



(ii) The Igneous Rocks. [A. M. D.] 



Two sills, each about 2 feet thick, penetrate the Tremadoc 

 Shales within a few feet of the principal fossil-bands, breaking 

 across the bedding within the diameter of the core. The effect 

 •of the intrusions on the sediments is slight, but those that 

 rest immediately on the sills are considerably baked and altered 

 into a hard dark-grey shale, which is also veined with calcite 

 and pyrite. The sills are so much alike, that one description 

 will serve for both. Each is a fine-grained, dull grey-green rock, 

 except for the very thin selvage which is pale purple, shading 

 off into white. The surfaces are flat, but have what might 

 be termed an ' upholstered ' appearance, there being depressions 

 resembling those produced on a stuffed textile fabric by sewing 

 down. There are many veins of calcite and pyrite, and on 

 some joint-surfaces pyrite is also abundant. On examination 

 with a lens, the rock an inch from the surface is seen to have a 

 finely-granular structure ; but towards the surface it becomes much 

 more homogeneous, except for what appear to be scattered porphy- 

 sritic crystals. These latter are found still in the thin surface-layer, 

 through which many of them appear to protrude : these protrusions, 

 together with hollows which are impressions of similar crystals, 

 impart to the surface a spotted appearance, in addition to the colour- 

 shading already mentioned. This, however, seems to be due to the 

 fact that the actual surface of the sill usually adheres to the shales, 

 so that what is seen is a surface of disruption within the thickness 

 of the selvage. 



In section under the microscope these sills show a very fine 

 ground-mass of small felspar-laths and granules of an opaque 

 mineral (magnetite ?). Scattered through this are (1) medium- 

 sized felspar-laths, about 0*5 mm. long ; (2) many rounded grains 

 which appear to be olivine-pseudomorphs, some in serpentine, 

 some in dolomite or magnesite, 1 a few in serpentine with a 

 dolomite or magnesite margin: the diameter of these varies from 



1 [This was originally described as calcite, but subsequent application of 

 Lemberg's test showed that it was not.] 



