Vol. 56.] BUNTEK PEBBLE-BEDS OF THE MIDLANDS. 303 



except that this latter mineral is more abundant, much resemble 

 some specimens from the Bunter. But, while admitting the diffi- 

 culty, I think that this negative argument ought not to outweigh 

 the large amount of positive evidence. 



Thus physical and lithological considerations seem to make it 

 highly improbable that the materials of the Midland pebble-beds 

 can have been derived from the south-western region, and impossible 

 that they can have come from any neighbouring district unless we 

 assume that great beds of conglomerate are buried out of sight 

 towards the east and south (which are wholly hypothetical), and a 

 very exceptional rainfall, for which we have no evidence. I have 

 taken some pains to collect my facts, have endeavoured to treat 

 them inductively, and am now content to leave the subject, unless 

 new discoveries compel me to reconsider my conclusions. 



EXPLANATION OF PLATE XX. 



(All figures X 18 diameters : 1 to 5 inclusive, from pebbles in the Bunter 

 Conglomerate.) 



Fig. 1. From Baland's Pool Pit. Spherulitic pitchstone (devitrified). The 

 spherulites are coloured with ferrite : the groundmass is speckled 

 with opacite, is perlitic in places, and shows, with crossed nicols, 

 minute devitrification-structure. The white spot at the bottom is a 

 hole in the slice. (P. 291.) 



2. From Style Cop Pit. Tourmalinized microgranite. Crystals of mica 



(probably biotite) replaced wholly or partly by tourmaline : the dark 

 lines, indicative of original cleavage-planes, being formed by microliths 

 of dull green tourmaline (schorl). The microcrystalline groundmass 

 consists mainly of quartz and tourmaline (brown and dull green), the 

 latter mineral doubtless generally replacing felspar. (P. 292.) 



3. From Style Cop Pit. Rhyolite with fluxion- and minute devitrification- 



structure, the former indicated by numerous minute lines of ferrite and 

 by larger, irregular, elongated streaks composed of tufted needles of 

 schorl. The white spots are mostly holes, but a very few small grains 

 of quartz, and some small crystals of felspar, partly converted into 

 tourmaline, occur in the slice. (P. 292.) 



4. From Baland's Pool Pit. A mica-felsite. The large grain on the 



right is brown tourmaline, containing some granules of iron-oxide ; that 

 on the left (partly torn away in grinding) consists of flakes of a rather 

 pale green chlorite, slightly tufted, but with a general disposition 

 parallel to the original cleavage of the mica. In other parts of the 

 slice needles of schorl are mixed with the chlorite, showing stages of 

 change from biotite to tourmaline. The devitrified groundmass 

 appears to consist mainly of felspar in ill-defined and rather irregular 

 granules. In this case, conversion into tourmaline seems practically 

 restricted to the biotite. (P. 292.) 



b. From a new-made road (probably from Racecourse Pit). Radiolarian 

 chert. The organisms are readily distinguishable ; the groundmass 

 exhibits a very minute chalcedonic structure, and is slightly speckled or 

 stained with ferrite. Very thin quartz-veins cross the slice. (P. 293.) 



6. Longmynds, near West Hill. Specimen most like a typical Torridon 

 Sandstone. This part of the slice is mainly composed of somewhat 

 rounded grains of volcanic rock, now apparently devitrified (varieties 

 of ' trachytes ') with a little felspar and some quartz. Though the 

 volcanic material varies in amount, it is everywhere dominant. 

 (P. 297.) 



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