NOUTFI-WEST KEGION OF CHAIJNWOOI) FOICKST. 95 



Wc oxamiiiod the neighbouring rooks on the other side of the 

 anticlinal, but obtained no additional information as to the repre- 

 sentatives of the lilackbrook Group. We found, however, a singular 

 rock hitherto unnoticed, a quarter of a mile south of Beacon Lodge, 

 on the west side of the road. It does not precisely agree with 

 anything else in the region, ])ut somewhat recalls the Sharpley rock, 

 and the resemblance is not macroscopic only. Microscopic exami- 

 nation sliows it to be composed of fragments of a devitrified 

 rhyolitic rock, exliibiting varietal differences of structure in rather 

 sharp contrast. These contain both crystals of felspar, showing some- 

 times oscillatory twinning, and grains of quartz. Larger grains of 

 the latter mineral also occur apparently not embedded in the matrix. 

 The rock undoubtedly has been modified by pressure ; its aspect on 

 the whole accords better with a pyroclastic origin, but if so, the 

 fragments were not at all scoriaceous. If it is a crushed (devi- 

 trified) '-obsidian," then we must assume an unusual amount of 

 flow-brecciation. 



10. Fragments and Pebbles. — Careful notes were made of the 

 nature of the fragments in the agglomerates, in the hope that they 

 might help in classifying the deposits. We distinguished four 

 varieties, designating them for our own purposes as (a) " Purple 

 Porphyritic " (rhy elite), (/3) " Compact Purple " (non-porphyritic 

 rhyolite), (y) Porcellanous (a " marbled," somewhat vitreous-looking 

 rock*), and (^) Syenitoid (a mottled rock with a slight superficial 

 resemblance to the syenite of the Porest). The first two are probably 

 different conditions of the same material, and are frequently found 

 together, so that there may be only three groups indicating sources 

 or epochs of discharge. We found that the fragments in any out- 

 crop more often belong to one only of these groups, but frequently 

 two, and sometimes all three, occur together. Slate fragments also 

 sometimes accompany them. The "purple" rock occurs everywhere, 

 and in particularly large masses in the spinneys east of Peldar Tor. 

 The " syenitoid " is the sole variety in the huge " rounded agglo- 

 merate" one-third of a mile E.N.E. of the Reformatory. The "por- 

 cellanous " occurs chiefly about Gun Hill, Cademan, and the 

 Whitwick-village (Pinfold) Quarry. The " porcellanous " and 

 " syenitoid " sometimes are not easily separated. If we take them 

 together it would appear that the group chiefly occurs either just 

 above and below the Sharpley rock (Gun Hill, Cademan), or on the 

 horizon it appears to occupy (in the" rounded agglomerate ") ; they 

 do not, however, occur at Ratchet Hill, where an agglomerate is 

 virtually in contact with Sharpley rock. Slate fragments are very 

 local ; they are large on the High-Towers Ridge and Swanymote 

 Rock, but occur in few other outcrops, and have not enabled us to 

 add to what we have already written. 



* The microscopic descriptions are given, though not under tliese separate 

 heads, in Part III. referring back to Part II. of our former paper. The names 

 apply only to the general aspect of the rock, and are not of any scientific 

 value. The "syenitoid" has a superficial resemblance to a holocrystalline 

 rock, but is not so really. 



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