308 LADY McEOBEET OX ACID AXD OTHEE [Dec. 1914? 



boundaries, with very few inclusions ; felspar- phenociysts are 

 entirely chalcedonized, and lie in a microcrystalline ground-mass 

 containing much spongy limonite. It is possible that this quartz- 

 porphyry occurs as a sheet running parallel to the contours, but 

 the nature of the exposure is such that it is impossible to speak 

 with certainty. The topmost rock of the hill, usually ending in an 

 abrupt escarpment, is a highly p o r p h y r i t i c s a n i d i n e - 1 r a c h y t e 

 with much quartz. It' consists of numerous fresh sanidine pheno- 

 crysts in a fine red ground-mass : the latter consists of sanidine- 

 laths, interstitial quartz, and limonite. 



No unaltered ferromagnesian mineral has been found in the 

 North Hill, but the habit and distribution of the limonite suggests 

 pseudomorphs after riebeckite. 



About 100 feet below the base of the laccolite on the west side 

 is a dyke of the same porphyritic trachyte. It seems to turn into 

 a sheet in the Old Red Sandstone, rising slightly eastwards, and to 

 crop out again in this form on the north and east sides of the hill. 



The igneous rock exposed on the col between the North 

 and Mid Hills is a porphyritic trachyte like that of the 

 lower part of the North Hill. 



In the Mid Hill similar porphyritic trachyte can be 

 recognized in the lower part of the northern scree-covered slopes 

 adjoining the col just mentioned. It may even extend con- 

 tinuously, perhaps as a basement sheet, to the more northerly of 

 the dykes jutting from the margin of the Mid Hill. This dyke, as 

 also its neighbour on the south, consists of sanidine -porphyry. 

 So far as one can judge on the scree- slope, the more southerly 

 porphyritic dyke extends for some little distance vertically into 

 the overlying felsite. There is, at the same time, the suggestion 

 that the porphyritic rock furnishes a bottom layer to the complex, 

 and reaches across the col between the Mid and Wester Hills. 



The porphyritic trachyte of these exposures is red and 

 much stained with limonite. It is rough to the touch, and 

 porous. Amygdales are sometimes abundant, and the jointing is 

 platy. Phenocrysts of fresh sanidine are found, but they are 

 more often pseudomorphosed in kaolinite or chalcedony. They lie 

 in a trachytic ground-mass of small fresh sanidine-laths, spongy 

 limonite, and much secondary quartz tilling vesicles and pores. 

 The quartz is sometimes idiomorphic, and is accompanied by 

 chalcedony. 



Closely similar porphyries, with a microcrystalline and felsitic 

 ground-mass, also occur. 



Much the greater part of the Mid and Wester Hills is 

 formed of riebeckite-f elsite (see map, PI. XLII). The 

 featuring of the two hills suggests that this rock occurs in two 

 layers. The upper layer shows magnificent columnar structure, 

 with very perfect hexagonal columns, on the south-western slopes 

 of the Wester Hill. 



The felsite (PI. XLI, fig. 2) is very hard and compact, and 



