480 ME. H. W. MONCKTON ON THE STIRLING DOLERITE. [Aug. 1895, 



32. The Stirling Dolerite. By Horace W. Monckton, Esq., 

 F.L.S., F.G.S. (Eead May 8th, 1895.) 



A glance at the Geological Map of Scotland will show that Stirling 

 €astle stands on one end of a very large mass of intrusive igneous 

 rock. Its continuity has been broken by a long east-and-west 

 fault a little south of St. Ninian's, but if we ignore this break the 

 mass of igneous rock has a length of about 8 miles and a width 

 varying from about | to nearly 2 miles. 



The rock has been intruded into strata of the lower part of the 

 Carboniferous Limestone Series, and it forms not one, but a number 

 of irregularly-shaped intrusive masses or group of sills, all more or 

 less connected with one another. The hardness of the igneous 

 rock has in many places been sufficient to arrest denudation, and 

 the result is a number of crags, which often resemble Salisbury 

 €raigs, near Edinburgh. Sometimes these crags face the north or 

 north-west, as in the case of Stirling Castle and King's Park, but 

 in other cases they face in a different direction, at Sauchieburn, for 

 instance, the north-east. 



As I shall show from the character of the rock, we can have 

 little doubt that the mass of the Abbey Craig north of the river 

 Forth is connected underground with the Stirling Castle rock, from 

 which it is only divided by alluvium, and there is reason to think 

 that the igneous rocks of Cowden Hill and of the hills around 

 Kilsyth are also outlying portions of the Stirling rock. It is not 

 meant that they are outliers, in the strict sense of that term, but 

 that there is an underground connexion between them. 



All these patches, as well as the main mass, are for the most 

 part composed of a more or less coarse-grained dolerite, the 

 marginal part always becoming finer grained and more basaltic in 

 character, and the actual edge has apparently been a tachylyte now 

 devitrified. 



Extensive quarries for road-metal, in the rock of the Abbey Craig 

 in the north and around Kilsyth in the south, enable one to obtain 

 specimens from the heart of the igneous rock, and I will take as 

 a first example a microscope-section from the rock at the bottom 

 of Wilson's quarry, Barrwood, Kilsyth. It is an ophitic dolerite 

 composed mainly of a plagioclase-felspar — probably labradorite, 

 and of augite, both in a fair state of preservation. The plagioclase- 

 crystals are long and narrow. There is a flecky green mineral, 

 probably a hydrous iron silicate, which is no doubt an alteration- 

 product after pyroxene, though in certain instances it has by some 

 process of infiltration succeeded in invading the felspar. There 

 is a good deal of iron oxide in irregular patches, and numbers of 

 clear colourless needles, sometimes -^ inch long, pierce the other 

 constituents of the rock. They are, I think, usually identified with 

 apatite. Small quartz-granules are scattered here and there, but 

 whether they are a normal product or foreign grains introduced 



