Vol. 51.] MR. H. W. MONCKTON ON THE STIRLING DOI/ERITE. 481 



into the rock I should hesitate to say. In the Geological Survey 

 explanation of Sheet 31 it is suggested that they may be derived 

 from arenaceous strata involved in the molten lava as it rose 

 upwards. 1 A little biotite occurs, often associated with the green 

 alteration-product. 



In a microscope-section from the uppermost rock in the same 

 quarry the felspar is still well preserved, but the crystals are 

 of smaller size. The pyroxene is replaced to a greater extent by 

 green mineral. 



In another microscope-section, from the quarry close to the 

 railway at Kilsyth, I noticed a crystal of hypersthene, and Mr. Teall 

 showed me one of his sections from near that town in which the 

 same mineral occurred. These are the only sections from the 

 Stirling dolerite in which I have seen it, though I have observed 

 altered crystals which look somewhat as though they had once been 

 hypersthene. 



Mr. Allport has figured a section of a rock from Gernal Brae, 

 near Kilsyth, which seems to be a rather fine-grained variety of 

 the rock that I am describing. 2 I call it ' fine-grained,' because 

 the largest plagioclase-crystal shown is only about the size of a 

 medium-sized plagioclase-crystal from the inner part of the rock at 

 Abbey Craig, or Kilsyth. A medium-sized crystal from the former 

 place measures 0-039 x 0-012 inch, and from the latter 0-055 x 0-012 

 inch. 



The igneous rock of Cowden Hill is of much the same character 

 as that of Kilsyth, but in such microscope-sections from that place 

 as I have examined the felspar is not well preserved. 



Passing now to the north of Stirling, the rock of the Abbey Craig 

 is very much like that already described. The iron oxide occurs in 

 rather large patches, and frequently in the form of skeleton-crystals 

 or masses broken up into globulites. In one case it seems to have 

 got into cracks in the felspar, and looks almost as if it had formed 

 after the felspar. 



Coming now to the main mass of the Stirling dolerite, I would 

 draw attention to some sections from Sir James Maitland's quarry 

 at Milnholm, 3 Sauchie. The rock is much the same as that of 

 Kilsyth and the Abbey Craig. The plagioclase-crystals are large, 

 sometimes 0'35 inch long, and very much altered into a flecky 

 white mineral ; the augite is well preserved. The iron oxide, biotite, 

 etc., occur as at Kilsyth, and there is a little hornblende intimately 

 associated with the augite. 



At several of the localities mentioned there are in the dolerite 

 pink or white patches and veins, and at the bottom of the Milnholm 

 Quarry the rock has a large amount of pink intermingled with the 

 dark greenish colour. A section from this part of the quarry shows 



1 Mem. Geol. Surv. Scotl. (1879) Expl. Sheet 31, p. 44. 



2 Quart. Journ. Geol. Soc. vol. xxx. (1874) pi. xxxiii. fig. 1. 



3 This quarry is about half-way between Craigend and Craigquarter — the 

 name is sometimes spelt ' Millholm.' 



2l2 



