116 DR HATCH ON THE LOWER CARBONIFEROUS 



the mass of the Garlton Hills had accumulated. No thick zones of tuff, nor interstrati- 

 fied layers of sedimentary rock can anywhere he seen, separating the numerous lava beds, 

 though it must be owned that the sections of the rocks are few and unsatisfactory." * 



The source of these streams of lava is indicated by the hills of Traprain Law and 

 North Berwick, and the Bass Rock in the Firth of Forth. These are regarded as vents 

 or "necks" by Sir Archibald Geikie. A microscopic examination shows that the 

 material filling these vents is of similar character to that composing the flows. 



The lavas are divisible into two distinct series. Of these the lower consists of 

 strongly basic rocks and forms a band extending from Traprain Law by Linton. White- 

 kirk, and Balgone to Fenton Tower, while the upper is a more acid (trachytic) series. 

 It forms the Garlton Hills and stretches away to the south between Whittingham and 

 Linplum.t 



I. The Lower, Basic Lavas (Basalts). 



The basic rocks vary, from a type rich in olivine, and almost entirely free from 

 felspar (limburgite), through ordinary olivine-basalts, to a more acid, strongly felspathic 

 type (labradorite-basalt). In this series the percentages of silica and magnesia vary 

 inversely. Thus the limburgite of Whitelaw Hill yields 40 per cent, of silica and 12 per 

 cent, of magnesia; the olivine-basalt of Kippie Law, 46 per cent, of silica and 6 '8 per 

 cent, of magnesia; the Hailes Castle rock, 49 per cent, of silica and 4*4 per cent, of 

 magnesia; while the labradorite-basalt of Markle Quarry contains 49 '5 per cent, of silica 

 and only 2 "8 per cent, of magnesia. 



The Limburgite of Whitelaw Hill. — Whitelaw Hill lies 4|- miles south-east of 

 Haddington. The material examined was obtained from Chester's Quarry. It is composed 

 of olivine, augite, and magnetic iron-ore.| Felspar is unrepresented, save by an occasional 

 skeleton-crystal. The augite is of a pale claret colour, deepening to violet at the margin 

 of the crystals. The olivine also is mainly unchanged in fresh specimens, but the course 

 of the alteration is indicated by the presence of a bright green pleochroic substance, 

 developed along the cleavage cracks. In more altered material the olivine is entirely 

 replaced by the same chloritic substance. 



The larger crystals lie in a ground-mass, which consists chiefly of an aggregate of 

 augite microlites, with intervening films and patches of a colourless glassy substance, 

 which in places is powdered over with a yellowish dust.§ Slender needles of apatite are 



* A. Geikie, he. eit. 



t Survey Memoir on East Lothian, p. 47, and Sir A. Geikie's Presidential Address already cited. 



X The name limburgite was first applied by Rosenbusch to the rock of the Kaiserstuhl in Breisgau. Limburgites 

 have since been described from numerous foreign localities, but are hitherto unrecorded in Great Britain. Quite 

 recently I have been able to note the occurrence of a similar type among the basic rocks of the Carboniferous volcanic 

 series in several places in Scotland besides that of Whitelaw Hill, e.g., Hill of Beath, Cowdenbeath (Fife) ; Pitandrew, 

 Fardel Castle (Fife) ; Southdean Law, 7 miles south of Jedburgh. 



§ A similar appearance is described by BoitlCKY as characteristic for his " lichte Magmabasalt." 



