482 MK. H. W. M0NCKT0N ON THE STIELING DOLEKITE. [Aug. 1895, 



the pink part to be a micropegmatite composed of a felspar and 

 possibly quartz, the former predominating. This micropegmatite 

 acts as a groundmass to the large crystals. These are plagioclase 

 usually much altered, augite well preserved, and a green mineral 

 in long crystals which may possibly be an alteration-product after 

 hypersthene. The iron oxide occurs both in small patches and in 

 long rods which in several cases attain a length of 0*1 inch. These 

 long rods do not occur in my other microscope-sections from the 

 Stirling dolerite. 



Mr. T. H. Waller 1 has described red and grey veins in the 

 Rowley Rag, but they do not appear to be of the character of the 

 red rock above noticed, for the felspar in the Rowley Rag vein is 

 described as of larger size than in the rock in the neighbourhood of 

 the vein, whereas in the Stirling rock the felspars of the veins and 

 pink patches are very minute as compared with those of the sur- 

 rounding rock. 



At one spot, a short distance south-west of the house at 

 Sauchieburn, the dolerite is strongly magnetic, so much so as to give 

 trouble to an engineer who was laying out a pipe-track, and who 

 was at first unable to account for the erratic behaviour of the compass 

 attached to his level. Even small specimens of this rock exhibit a 

 distinct polarity, one side showing a tendency to attract the north 

 and the opposite side the south point of the magnetic needle. A 

 microscope-section shows that, excepting for its magnetic properties r 

 the rock is of the ordinary character. The felspar is well preserved, 

 and there is a little brown hornblende associated with the augite. 



The more coarsely crystalline part of the Stirling rock has been 

 now sufficiently described, and I pass on to consider the finer- 

 grained, often almost glassy, margin of the intrusive sheets or sills. 



This change of character as one approaches the margin has been 

 very fully and accurately described by Sir Archibald Geikie, 2 and I 

 have little that is new to add to his account. The following notes, 

 however, may be of interest. 



On the western side of the Sauchie mass of igneous rock there is 

 a magnificent natural section along the valley of the Bannockburn. 3 



Section of Sauchie Craig. 



1. Intrusive dolerite forming the top of the Craig ; maximum thickness about 



100 feet. 



2. Carboniferous Limestone Series, consisting of shales and sandstones with 



one or two thin limestones, known as the Hurlet Limestone. About 

 130 feet. 



3. Calciferous Sandstone Series, not well shown. About 50 feet. 



4. Porphyrite. Interbedded with the Calciferous Sandstone Series, shown in 



numerous sections along the Bannockburn. 



1 ' Midland Naturalist,' vol. viii. (1885) p. 261. 



3 ' Carboniferous Volcanic Rocks of the Firth of Forth Basin,' Trans, Roy. 

 Soc. Edinb. vol. xxix. (1879) p. 496. 



3 See the diagrammatic section through Sauchie Craig, Proc. Geol. Assoc, 

 vol. xii. (1892) p. 251. 



