556 SECTIONAL TRANSACTIONS.— C. 



Dr. W. Mackie. — Preliminary Report on the Heavy Minerals of the 

 Silurian Rocks of Southern Scotland. 



About fifteen specimens of greywacke were collected in the spring of 1926 from 

 an area round the town of Peebles of about two miles' radius, and the heavy minerals 

 isolated from them gave unexpected results. All of them were found to be ' flooded ' 

 with particles of a fine greenish to greenish-yellow augite, which often showed 

 characteristic inclusions. Along with these was a proportion of hornblende and 

 enstatite fragments, but these were always in relatively much fewer numbers. These 

 residues also contained garnet, sphene, and almost always zircons, both coloured 

 (purple) and colourless with, in addition, some apatites, epidotes and tourmalines. 

 In these were also observed numerous minute angular fragments of a glossy dark 

 brown colour often exhibiting conchoidal fracture, and which were found to be 

 uniformly isotropic, but they were so insignificantly small that their true import 

 was at first overlooked. They were later found to occur in marked excess in the rocks 

 to the south-west of the area, and have since been referred to the mineral melanite. 

 A fairly large number of specimens have since been collected and their content of 

 heavy minerals specifically determined, but up to the present these cannot be said 

 to represent more than a small fraction of the entire area, but typical specimens have 

 been examined at intervals along the northern margin of the area from Peebles to 

 Ballantrae and from the latter locality across the strike to the Rhinns of Galloway 

 and Glenluce. Occasional specimens from the rest of the area, chiefly for the purpose 

 of comparison, have also been examined. 



Over all the results from sixty-one specimens have been tabulated with the 

 following results : — 



Thirty-four out of 61 showed the presence of augite, 22 hornblende, 14 enstatite, 

 52 zircon (of which 34 contained purple zircons), 38 garnets, 19 sphene, 38 melanite,' 

 5 glaucophane, 21 epidote, 23 apatite, 16 tourmalines, 18 rutile, 7 pyrite, 11 chlorite, 

 2 anatase and 1 each of brookite, magnetite, dolomite, fluor and hypersthene. 



The feature of the minerals generally is their remarkable freshness, which is 

 evidently due to the relative impermeability of the Silurian rocks. Augite is not a 

 mineral that figures largely in heavy mineral residues, while melanite is probabl}^ 

 unique in its occurrence, both as regards extent and quantity. Both of these minerals 

 are very widely distributed, but it is doubtful if they were originally associated in 

 one and the same rock mass becaiise melanite is present often in abundance where 

 augite has not so far been found. They occur in such abundance and over so wide 

 an area that the natural inference is that they were derived from rock masses of 

 considerable superficial extent outside the existing area of Silurian rooks. 



The above list of minerals is also interesting from the negative side — monazite 

 is so far entirely absent from the list. There is also no kyanite, staurolite, silHmanite 

 or andalusite. 



It may be somewhat premature to attempt to indicate the possible origin of the 

 Silurian sediments, but a large area of basic volcanic or plutonic rocks is suggested. 

 The Ben Ledi grits evidently came into the area, as may be inferred from the frequent 

 presence of chlorite, tourmaline and purple zircons- in association, these being the 

 characteristic minerals of the Ben Ledi grits, while the only known occurrence of 

 glaucophane in mid-Scotland is in these rocks. From the number of characteristic , 

 colourless and generally unworn zircons that appear locally in the south-west of the 

 area, a foliated granite-' (or granites) also appears to be indicated. 



Mr. E. H. Davison. — The Geology and Economics of the West of England 

 China-clay Deposits. 



China clay has been worked in Cornwall and Devon since the beginning of last 

 century, and the industry has now developed to such an extent that 800,000 tons 

 were produced in 1927. 



The china clay rock from which the china clay is obtained, occurs in all the granite 



' It has been determined that the mineral is not melanite. What it is has not 

 so far been ascertained. 



2 'The source of the Purple Zircons in the Sedimentary Rocks of Scotland.' 

 Trans, of Edin. Geological Society, vol. xi, part ii, p. 200. 



8 ' The Heavier Accessory Minerals in the Granites of Scotland.' Edin, Geological 

 Society, vol. xii, part i, p. 22. 



