Vol.51.] ME. H. W. MONCKTON ON THE STIELING DOLEKITB. 489 



(v) At the bottom of the microscope-section — that is, 0*7 inch from 

 the top of the igneous rock — minute laths of felspar are 

 beginning to appear in the groundmass in great numbers. 

 The porphyritic felspars as before. 



The whole structure of the uppermost part of this rock is 

 suggestive of rapid cooling, and the microscope-section 3-7 inches 

 from the top shows the porphyritic rock which we seem always to 

 find between the basaltic edge and the dolerite-centre of the intrusive 

 sheet. 1 



In the other microscope-section, 3"70 inches below the top, the 

 groundmass is composed of lath-shaped felspars, and there may be 

 a little minute granular augite ; but if so, it is in very small 

 quantity. The porphyritic mineral is plagioclase as before. The 

 iron oxide is in the form of small globules and minute rods, which 

 latter are collected in groups and cross one another usually at 

 right angles, forming a sort of trellis-work. 2 



A short distance north-west of this locality, at the point marked 

 4 on the sketch-map, there is a small quarry in which the igneous 

 rock is of an amygdaloidal character. 



At the surface of the ground there is a little shale, and the top 

 edge of the igneous rock is very well shown. I have a microscope- 

 section taken 5| feet below the junction. The rock is not por- 

 phyritic, though the plagioclase-crystals are of variable size (O015 

 X 0-004 is an average measurement). There is a certain amount of 

 augite which has been much altered, and some dark-coloured mica. 

 A good cross of felspar is seen close to one edge of the slide. 

 There is a tendency on the part of the felspars to arrange themselves 

 in star-shaped groups. The iron oxide occurs in patches and 

 globules and in minute rods, which are frequently long and hair- 

 like. These rods and hairs are often arranged in patches, one of 

 which measures 0-09 x 0*05 inch ; the hairs very often have a small 

 knob at one end. The amygdules, which are numerous, consist of a 

 green mineral, which is probably serpentinous, and iron oxide. 



The jointing of the rock is nearly vertical, and in places the 

 joints are continued into the overlying shale for 6 inches or more. 

 The top of the rock is very r irregular, and small patches of stratified 

 rock remain in the hollows of the igneous mass, having been thereby 

 protected from denuding agents. There is a little Boulder Clay 

 at the surface, and under it the rock is ice-marked. At the west side 

 of the quarry the altered shale contains fossils, such as Productus, 

 etc., and is overlain by an encrinital limestone 3 inches thick. 



West of this quarry I found in one place a vein of dolerite, 

 0*40 inch thick, running through the basaltic marginal portion of 

 the mass of igneous rock. 



At the points 10 and 11 on the sketch-map there are small 

 exposures of basalt at a much lower level than the main mass of 

 the igneous rock, and they are probabby connected with the lower 



1 A. Geikie, Trans. Eoy. Soc. Edinb. vol. xxix. (1879) p. 49G. 



2 Ibid. pi. xii. fig. 12. 



