122 



DR HATCH ON THE LOWER CARBONIFEROUS 



mass of minute spicular microlites of the same mineral ; but in other rocks (e.g., Pencraig 



Quarry *) there is less tendency to develop lath-shaped crystals, the granular form taking 



its place. In others, again (e.g., at Craigie Hill), a passage to the porphyritic trachytes 



is produced by the sporadic appearance of sanidine crystals belonging to an earlier 



generation. 



Chemical Analyses of the Trachytes.^ 





Peppercraig, 



Sect. No. 615 



(Wilson). 



Hopetoun 

 Monument, * 

 Sect. No. 620 

 (G. Barrow). 



Kae Heughs, 



Sect. No. 635 



(Wilson). 



Phantassie,* 

 Sect. No. 622 

 (A. Dick, jun.). 



Bangley 



Quarry, * 



Sect. No. 625 



(A.Dick,jun.). 



Si0 2 

 A1 2 3 

 Fe 2 3 

 FeO 

 MnO 

 CaO 

 MgO 

 K 2 

 H 2 

 Loss on igni- > 

 tion J 



62-61 



18-17 



0-32 



4-25 



•21 



2-58 



•74 



4-02 



6-49 



•80 



62-50 

 18-51 



| 4-39 



2-00 



•61 



6-31 



3-44 



2-10 



61-35 



16-88 



•41 



5-01 



•26 



2-39 



•44 



6-12 



5-26 



1-70 



59-50 



18-25 



4-81 



2-34 



2-10 



•70 



6-30 



5-03 



1-60 



58-50 



21-12 



4-68 



370 

 •93 



5-84 

 3-90 



2-00 





10019 



99-86 



99'82 



100-63 



100-67 



Sp. G. 



2-6 



... 



2-6 







III. The Volcanic Vents. 



The position of the vents, from which the lavas of the Garlton Hills took their source, 

 is indicated by the presence, on the margin of the volcanic area, of masses of agglomerate 

 and plugs of igneous rock. Some of these are exposed along the coast of the Firth of 

 Forth, between Dunbar and North Berwick ; others form good-sized hills, such as North 

 Berwick Law (612 ft.), on the north of the district, and Traprain Law (724 ft.), on the 

 south. The Bass Rock (350 ft.) marks the site of another " neck." 



A portion of the earliest (basic) lavas doubtless flowed from vents situated near 

 Dunbar. Thus the knob of olivine-basalt on which Dunbar Castle stands, is probably the 

 exposed core of one of the pipes of emission. This rock proves to be very basic, being 

 composed of numerous porphyritic crystals of olivine, imbedded in a brown crypto- 

 crystalline ground-mass, containing grains of augite, but not much felspar. Again, a 

 limburgite, very similar to that which forms Whitelaw Hill, occurs as an intrusive mass 

 in tuff at Gin Head, Tantallon. 



* Figured by Sir Archibald Geikie, loc. cit. 



t These analyses were kindly made for me by Messrs G. Barrow and A. Dick, jun., in the Laboratory of the 

 Geological Survey at 28 Jermyn Street, London. 



