484 ME. H. W. MOXCKTON ON THE STIKL1NG DOLEKITE. [Aug. 1 89 5, 



shows porphyritic felspars (which are much decayed) in a dark 

 grey groundmass, consisting in the upper part of the slide of minute 

 felspar-crystals, but lower down being almost wholly composed of 

 iron oxide. There is no augite, nor do the iron -oxide rods or hairs 

 of the last microscope-section appear. A band about O08 inch thick 

 at the bottom of the igneous rock is marked by what may perhaps 

 be an approach to a spherulitic structure ; that is to say, there is a 

 tendency on the part of the granules of iron oxide to form small 

 globular aggregates. 



So far as I can judge from an inspection with a lens, the same 

 class of change takes place all along the margin of the igneous rock 

 in Sauchie Craig, and I have some microscope-sections from Cowden 

 Hill which show similar characteristics. 



An outlying patch * of igneous rock at Touchadam Quarry, 150 

 yards west of the Sauchie Craig mass, and on the opposite side of 

 the Bannockburn, furnished me with another specimen showing 

 the junction of the igneous rock and underlying white altered shale. 

 A microscope- section cut through the junction shows in the upper 

 part of the slide a grey groundmass with porphyritic plagio- 

 clase-crystals which are much changed. Here and there are 

 granules of quartz, associated sometimes with iron oxide. The 

 quartz is probably secondary, filling up minute vesicles, on the edge 

 of which it is possible that the iron oxide formed in cooling. An 

 examination of the rock shows that this porphyritic portion is several 

 inches thick. At the bottom of the igneous rock there is a zone, 

 about 0*11 inch thick, of very hard black rock, with an indistinct 

 spherulitic structure. The groundmass is in places formed into 

 irregular lines showing a flow-structure along the bottom of the 

 rock. Sometimes the porphyritic felspars are more or less parallel 

 to the bottom, but they are often set at various angles and the 

 lines of flow curve in and out amongst them. These felspars 

 frequently contain enclosures of iron oxide. The line of junction 

 is irregular, but well defined. At one point a portion of the under- 

 lying rock is partially enclosed in the basalt. The underlying rock 

 is a white indurated shale ('calm'). Granules of quartz are scattered 

 through it, and along the junction are collected together in places. 



I now cross over to the eastern side of the Stirling mass of 

 igneous rock, in order to describe its contact with the overlying 

 strata, and I have, with the assistance of the 6-inch map, prepared 

 a sketch-map of the locality (fig. 2, p. 486), to which I more 

 especially wish to draw attention. 



Between the points marked 1 and 2 on the sketch-map there are 

 some exposures showing the igneous rock and its contact with 

 overlying shale. Some specimens from locality 2 were described by 

 Mr. J. (x. Goodchild in an appendix to my former paper, and I am 

 much indebted to him for assistance in working out the geology of 

 this district. 2 



1 This patch is shown on the 6-inch, hut not on the 1-inch Geological Survey 

 map. 



2 Proc. Geol. Assoc, vol. xii. (1892) p. 252. 



