Yol. 51.] MR. H. W. MONCKTON ON THE STIRLING DOLERITE. 483 



At the northern end of the Craig are some disused limekilns, and 

 above them the junction of the igneous rock and the underlying 

 strata is very well shown. 



The strata are inclined at a low angle, and the bottom of the 

 igneous rock, which is nearly horizontal, cuts across the bedding. 

 At the kilns the level of the bottom of the igneous rock is about 

 400 feet O.D., and 60 or 70 feet above the Bannockburn. 



The Hurlet Limestone has in former times been worked under 

 the igneous rock, and close to the mouth of an old level (part of the 

 old workings) the bottom of the igneous rock descends abruptly 

 through the strata. It soon rises again, however, and at a place 

 locally named the Goat's Mouth x it has risen to a level of 600 feet 

 O.D. : that is, some 200 feet higher than at the limekilns. 



The jointing of the igneous rock is almost vertical or slightly 

 inclined to the south. In one place a prominent vertical vein of 

 ealcite runs through it, and just south of the vein the igneous 

 rock again rises up through the strata, giving good evidence of its 

 intrusive character. 



For a long distance southward excellent sections showing the 

 junction of the igneous rock with the underlying strata are to be 

 seen, but it is unnecessary to describe them in detail. At one 

 place a mass of strata about 12 feet thick is partially enclosed 

 in the igneous rock. That is to say, it has about 8 feet of igneous 

 rock below and about 30 feet of the same rock above it. It consists 

 of the very much altered whitish shale locally known as ' calm.' 



The greater part of the igneous rock is the more or less coarse- 

 grained dolerite, which we have been describing from other parts 

 of the mass, but as we approach the edge a change takes place, and 

 the grain becomes gradually finer. A microscope-section taken 

 5 inches from the bottom shows : 



(a) That the rock is beginning to assume a porphyritic character, 

 some of the crystals of plagioclase being of larger size than 

 the others. 



(6) There is no augite to be seen, though there is a little green 

 mineral which is perhaps an alteration-product after augite : 

 probably the rock solidified just as the augite was beginning 

 to come out of the groundmass. 



(c) There seems to have been once a certain amount of partially 



individualized groundmass, perhaps even a glassy base in 

 the rock. 



(d) There are collections of straight hair-like iron oxide which 



sometimes form a network, and small hairs and rods of iron 

 oxide are scattered through the slide, in the manner shown 

 by Sir Archibald Geikie in one of his sections. 



(e) The rock effervesces freely with cold hydrochloric acid. 



This rock becomes slightly more porphyritic as it gets nearer the 

 bottom, and a section cut perpendicularly through the bottom 



1 Name not on the maps. 



