340 DR. A. M. DAVIES AND ME. J. PEINGLE ON [June I913,. 



The following are the chief suggestions of a more theoretical 

 character : — 



(1) That the gas-bearing strata of the "Western Boring are 

 Triassic, the actual margin of that system lying between the two 

 borings at Calvert. (The alternative possibility that those strata 

 are Upper Palaeozoic is considered.) 



(2) That in the Bletchley Boring Oxford Clay, Forest Marble, 

 and Lower Lias are present. 



(3) A contoured map of the Palaeozoic floor is presented, as a 

 basis for future discussion. 



(4) Suggestions are made as to the structure and composition of 

 the Palaeozoic floor. 



EXPLANATION OF PLATES XXXIII & XXXIV. 



Plate XXXIII. 



Microscope-sections of rocks from the Calvert Eastern Boring. 



Fig. 1. Bottom bed of Great Oolite, at 195 feet : X 23. In the centre is a 

 rounded fragment of oolitic limestone, with a coating of calcite, 

 dusky marginally, and partly worn away. Derived oolite-grains with 

 dusky margins are scattered through the matrix. (See p. 318.) 



2. Sandy beds in Chipping-Norton Limestone, at 201 feet : X 23. In 



the centre is a subangular derived fragment of oolitic limestone with 

 dusky margin. (See p. 319.) 



3. Same beds : X 17. In the centre is an oolite-grain indented by angular 



quartz-grains ; around are oolite-grains and fragments, with dusky 

 margins. (See p. 319. ) 



4. Olivine-basalt sill at 606 feet : X 20. Porphyritic olivine-pseudo- 



morphs in calcite, showing parallel growth and zonal inclusions ; 

 groundmass with felspar-laths and minute opaque granules. (See 

 p. 326.) 



5. Selvage of the same sill : X 22|. Margin on the right ; across the 



centre a calcite-vein ; the dark patch below this is probably an in- 

 clusion of shale; olivine-pseudomorphs in calcite; felspar-laths. 

 (See p. 326.) 



6. Portion of the same : X 90. Felspar-laths ; vesicles (?) filled with 



chlorite ; groundmass. 



Plate XXXIV. 



Map of part of South-Central and South-Eastern England, on the scale of 

 10 miles to the inch, or 1 : 633,600. 



The contour-lines give the estimated depth of the base of the marine Mesozoic- 

 strata (Gault or Jurassic) aboTe or below Ordnance datum. To the south-east. 

 of the dotted line (approximate south-eastern limit of Trias) they are also 

 contour-lines of the Palaeozoic floor. 



The localities to which the numbers on the map refer are enumerated in the 

 table on p. 339. The figures at the side, if not followed by a letter, denote the 

 depth below O.D. of the Palaeozoic floor immediately beneath marine Mesozoic 

 strata ; when followed by a , they denote the depth below O.D. at which 

 marine Mesozoic strata rest upon Trias ; when followed by a J, they denote 

 the depth below O.D. at which the boring ended in Jurassic strata. 



