1 16 Transactions of tJie [Sess. 



by walls of basalt intruded among the carboniferous strata of the 

 district. The soft sandstone and shales have been removed by the 

 denuding forces of rain, frost, &c. The hard basalt offers more 

 resistance, and forms the headlands and hills along the coast. Some 

 good sections of basalt were seen in the quarries, showing the char- 

 acteristic double system of jointing, each block weathering in con- 

 centric spheroidal coats, and changing in colour from black to yellow 

 as its iron oxide becomes transformed into the hydrous peroxide 

 limonite. Some of the newly exposed surfaces were slicken-sided. 

 The variety of basalt is a fine-grained greenstone, generally con- 

 taining grains of iron pyrites. A little west of St Davids the car- 

 boniferous strata appear — finely laminated black shale overlying 

 fine-grained sandstone; these dip about 20° N.W. under the 

 greenstone. The ballast heaps presented a wayside museum of 

 schists, sandstone, granites, and flints, which probably once formed 

 part of the shores of the Highlands or Scandinavia. 



EAST LINTOK AND PEESSMENNAX— /?«ic 3, 1882. 



The high-road between East Linton and Pressmennan lies over the 

 carboniferous strata, which extend through the lowland valley from 

 the coast of Ayrshire to Dunbar. Fei'tile fields lie on either side, 

 and these owe their fertility not so much to the immediately under- 

 lying rocks as to glaciers of the Ice Age which ground down the 

 face of the country over which they passed, and carried with them 

 a happy mixture of sand, clay, and lime. The intrusive volcanic 

 rocks of the neighbourhood — e.g., Traprain and Berwick Law — give 

 diversity to the scenery because of their hardness. The sandstones 

 have been worn away, and the basalt, weathering equally all round, 

 has assumed a shaj5e somewhat like a true volcanic cone. 



Pressmennan Loch lies in a hollow, bounded by steep rocky 

 banks and closed in at each end by rocks. Its formation has not 

 yet been explained. The brooks in the neighbourhood are not large 

 enough to have hollowed out such a basin, and its form is not such 

 as would have resulted from glacial action. 



CRAIGMILLAR— J^M?w 7, 1882. 

 The high-road between Powburn and Craigmillar again lies over 

 carboniferous sandstone, red and white. At a small quarry an in- 

 teresting example of slicken-sides was seen. The sandstone had 

 been removed down a line of jointing for several yards. Between 

 the sides of the joint water containing a mineral had passed, enam- 

 elling the surface. The grinding together of the rock along the 

 line of jointing had produced the usual fine striations. The castle 

 is situated near the summit of a hill formed by an anticline. The 

 strata on which it rests are nearly horizontal, and dip away on 

 either side. 



