608 REPORT — 1886. 



micrographic arrangement of quartz and felspar. In Cornwall, among beds which 

 are almost certainly Ordovician or Silurian, we find similar evidence of derivation 

 from much more ancient rocks. The conglomerates of the Meneage district con- 

 tain, in addition to quartzites, grey wackes, and other old sedimentary beds, abundant 

 fragments of a moderately coarse-grained granitoid rock, and occasionally a horn- 

 blendic rock similar to the well-known Lizard schist. A series of specimens which 

 [ have examined microscopically shows, in addition to compact igneous rocks, 

 apparently volcanic, quartz grains probably derived from granitoid rock, various 

 fine-grained schists and schistose argillites or phyllites, quartzites, grits, and other 

 older clastic rocks. One fragment of schist contains little eyes of felspar, and in 

 general structure reminds me of some in the so-caUed * Upper Gneiss ' series of 

 N.W. Scotland. Another, a fine-grained mica-schist or a phyllite, exhibits a 

 cleavage transverse to the rumpled foliation. 



A rich harvest probably awaits the explorer in the ' greywackea ' of the 

 southern uplands of Scotland. A ' Lower Silurian ' conglomerate from Kingside, 

 Peeblesshire, contains numerous fragments of igneous rocks, probably of volcanic 

 origin, and bits of granitoid rock, with some which are either very old quartzites or 

 perhaps vein-quartz. These have been crushed and re-cemented before being 

 detached from the parent rock. The basement conglomerate of the Craig Head 

 limestone group (Llandeilo-Bala) is full of rounded fragments of volcanic rocks. 

 These, as in the last-named case, exhibit considerable variation ; the majority, how- 

 ever, are probably andesites, and perhaps in one or two cases even basalts. A 

 Middle Llandovery conglomerate from near Girvan is largely made up of frag- 

 ments which appear to have been derived from very ancient quartzose rocks. The 

 greywacke of rather later age from near Heriot, Edinburghshire, contains, with 

 numerous volcanic fragments, and a little argillite, a few bits of fine-grained quartz- 

 schist, together with grains of quartz and felspar, suggestive of derivation from a 

 more coarsely crystalline rock. 



Old Red Sandstone and Devoyiian. — It is, I believe, indisputable that when the 

 Old Red Sandstone of Scotland was formed a great period of mountain-making had 

 ended and one of mountain-sculpture was far advanced. The conglomerates are 

 often full of fragments of the crystalline rocks of the Highlands, and no doubt the 

 sandstones derived their quartz-grains from the same source. In the southern 

 half of the country, however, as is well known, volcanic materials, more or les» 

 contemporaneous, play an important part. I have not been able to examine closely 

 the Old Red Sandstones of England and Wales, but their frequent near resemblance 

 to the sandstones of Scotland suggests a similar derivation. True, the materials 

 may have been sifted from older clastic rocks, but there is nothing specially to 

 suggest this, and the abundant pebbles of vein-quartz, which I have seen in one or 

 two localities, seem rather more favourable to the other hypothesis. I have only 

 examined microscopically a very few specimens of Devonian grit, all from the 

 south side of the county. These certainly seem to have derived their materials, in 

 part at least, from crystalline rocks, both granitoid and schists of finer grain ; one 

 specimen also apparently containing some bits of hypometamorphic rock. 



Carboniferous. — In Scotland some of the basement beds of this series so closely 

 resemble the Old Red Sandstone that no further description is needed, and the same 

 remark may be made of the very few overlying sandstones which I have carefully 

 examined. In the North of England the basement conglomerates, so far as I have 

 seen them, are made up of earlier Palaeozoic rocks, but for many of the great 

 masses of sandstone which occur in the series a source of supply is not so easily 

 found. Dr. Sorby, who has made a special study of the Millstone grit of South 

 Yorkshire, tells us that it is formed of grains of quartz and felspar, apparently 

 derived from a granite, and contains pebbles, sometimes an inch or so in diameter, 

 of vein-quartz, of hard grits, of an almost black quartz-rock or quartz-schist, and of 

 a non-micaceous granite. He also notes one fragment of a greenstone, and another 

 either of a fine-grained mica-schist or of a clay-slate. The granite, he states, 

 more resembled those of Scandinavia than any one now visible in Britain, and the 

 bedding indicated a supply of materials from the north-east. In the Millstone 

 grit near Sheffield he says that the grains appear to be but little worn, as if they 



