Thomson. — Flint-beds associated irifh Amiiri Limestone. 55 



There is reason to suspect that this specimen came from the band of flint- 

 carbonate rock intercalated near the top of the nuidstones in this section, 

 for this band exhibits much larger rhombs than usual. 



A qualitative chemical test, carried out by Mr. B. G. Aston, showed 

 that the carbonate of the coarsely crystalline specimen from the Isolated 

 Hill Creek was strongly magnesian. A specimen from the Ure River 

 gravels of the normal type of fiint-carbonate rock, containing little flint 

 matrix, was then submitted to the Dominion Analyst, Dr. Maclaurin, for 

 analysis, with the following result : — 



SiOo .. .. .. 26-18 



Al,03+Fe203 



Mi^O 



CaO 



H.,0 + . . 



H2O— .. 



CO, 



Undetermined 



100-00 



This analysis reveals two interesting facts. In the first place, it shows 

 that the carbonate crystals have approximately the composition of dolomite. 

 It seems most probable that they are pure dolomite, in view of the known 

 power of crystallization of that mineral, and that the excess of calcium 

 carbonate over the dolomite molecule is contained in the matrix. In the 

 second place, it shows that the silica of the flint is little hydrated, and is 

 not opal. In microscopic section the flint matrix is nearly dark between 

 crossed nicoLs, but often shows numerous very small laths with moderate 

 birefringence, straight extinction, and positive elongation, probably one 

 of the orthorhombic forms of silica. They are altogether too minute for 

 study in convergent light with the apparatus available. 



The flint-beds at the base of the flint series in Sawpit Gully, Coverham, 

 differ from those higher up in the series in that they contain a large 

 amount of clastic quartz and other minerals in a matrix similar to that 

 of the more normal flint-beds. The quartz is the most plentiful of the 

 included minerals, and is in small angular grains. Muscovite in thin 

 flakes appears in considerable amount, while there are scattered grains of 

 glauconite and numerous minute crystals of rutile and pyrite with some 

 iron - hydroxide staining. There are small rectangular areas of finely 

 crystalline silica, often containing sericite, which possibly represent a 

 replacement of clastic grains of feldspar. The light-coloured exteriors of 

 these flints have a similar composition, except that there is less of the flint 

 matrix and a greater proportion of subparallel plates of muscovite. There 

 seems little doubt that in this case the flint-beds are replacing an impure 

 fine-grained sandstone containing a little glauconite. 



The flinty band intercalated in the Cenomanian mudstones of the Nidd 

 Stream, above Sa\\^it Gully, is similar in composition and structure to 

 the last described, with the difference that the minerals enclosed in the 

 flint matrix are about half as large, and include very little glauconite. 



IV. Origin of the Flint-beds. 



Chert-beds not occurring in association with limestones or dolomites 

 are usually the result of the molecular rearrangement in situ of silica derived 

 from skeletons of such organisms as Radiolaria, sponges, or diatoms. The 



