490 ME. H. W. MONCKTON ON THK STIRLING DOLERITE. [Aug. 1 895, 



sheet which runs out from quarry 4 to localities 6 and 9. The 

 rock is of the usual marginal character, and at locality 11 the top 

 is ice-marked. 



Between these lower basalts and the main sheet there is a con- 

 siderable thickness of strata through which the Hosie's Limestone 

 runs, and it has been worked by levels driven under the main sheet 

 of igneous rock at the point marked ' Old lime-workings ' on the 

 sketch-map. 



From near these lime-workings a thin sheet of igneous rock 

 runs out past Sauchie Castle, and in the burn there an excellent 

 section is exposed. There is at the top some 20 feet of dolerite, 

 with the usual basaltic band or zone at the bottom. Below it 

 there is some 7 feet of black shale, and below this again a band, 

 3 feet thick, of very hard rock which seems to be a black calcareous 

 shale much altered and indurated ; fossils can, however, still be 

 distinguished in it. Below this bed come 6 feet of shale and then 

 a sandstone, white and indurated, apparently, by the overlying 

 igneous rock. This seems here to have affected the beds to an 

 unusual distance below it. The underlying shales, sandstone, lime- 

 stone, and coal do not appear to have been affected. 



In connexion with the contact-alteration I may mention a 

 microscope-section from a white altered shale at Cowden Hill 

 in which there are numerous small coDections or radial bunches of 

 a white mineral which polarizes in brilliant colours. These col- 

 lections in the contact-shale remind one of the spherulitic appearance 

 of the contact-basalt. 



I have in the preceding pages described contact-specimens from 

 the bottom of the main mass of igneous rock at Sauchie Craig and 

 from the outlying patch at Touchadam, also from the top of the 

 main mass at Sauchieburn (locality 2 on the sketch-map) and from 

 the top of minor sheets at localities 8, .6, and 4, and in all these cases 

 there is seen to be a series of changes which take place as we 

 approach the margin. The sequence seems to be somewhat as 

 follows, beginning from the middle : — 



1. Coarse-grained ophitic dolerite, which forms the greater part of 



the intrusive rock. 



2. Fine-grained ophitic dolerite, say 10 to 20 feet from the margin. 



3. Basalt, with very little augite and with hairs and rods of iron 



oxide in groups and scattered through the rock. 



4. Basalt, with porphyritic plagioclase-crystals in a groundmass 



consisting of microliths of plagioclase and of iron — hairs and 

 rods of iron oxide ; no augite. This occurs as near as \ inch 

 to the junction and as far as 5 inches from it. 



5. Basalt, with porphyritic plagioclase in a grey groundmass, say 



yL to 1 inch from the margin. 



6. Basalt, with porphyritic plagioclase in a hard groundmass, some- 



times spherulitic — about ^ inch thick on an average. 1 



1 Compare Sir A. Geikie. ' Carboniferous Volcanic Rocks of the Firth of 

 Forth Basin,' Trans. Roy. Soc. Edinb. vol. xxix. (1879) p. 496, pi. xii. fig. 12 ; 

 Zirkel, ' Basaltgesteine,' Bonn, 1870, p. 92 ; Vogelsang, ' Die Krystalliten,' 

 Leipzig, 1874, pi. xiii. fig. 1 ; Teall, ' British Petrography,' 1888, p. 166. 



