Vol. 70.] IXTEUSIOXS >'EAE MELEOSE. 307 



The Eildon Hills are probably the denuded remains of a 

 composite laccolite. The North Hill is petrographically distinguish- 

 able, as Mr. Barron has pointed out, from the Mid and Wester 

 Hills, and was probably a subsidiary dome. As the laccolite lies 

 far above the plane of local erosion, its entire sedimentary cover 

 and a good part of the igneous core have been removed. At the 

 present time some 300 or 400 feet of the latter remain. There is 

 no representative of the roof except, perhaps, the narrow band 

 of Old Red Sandstone which is exposed in the quarry between 

 Mid and North Hills. Probably only the eastern half of the 

 laccolite is preserved, as is indicated by the outward dip of the 

 rocks in the north-eastern and south-eastern quadrants. The 

 present western boundary and its three feeders represent the 

 central part of the original laccolite ; and the small outlier of 

 Bowden-Moor Quarry, with its strip of underlying Old Red Grit, 

 is probably the sole remnant of the western half. 



That the igneous rocks are in the form of thick sheets or a 

 laccolite, rather than a plug, is inferred from the general appear- 

 and of the mass ; but clear exposures, showing that it has a base, 

 are nowhere available. That portions, at least, of the complex are 

 intrusive is clear from the following considerations : — ■ 



A narrow strip of igneous rock connects the North and Mid Hills, and it is 

 seen that this strip, in part at least, has dyke-like relations, for the igneous 

 rock cuts steeply through Old Red mudstone exposed at the brink of a 

 small quarry. The chilled edge against the mudstone is remarkable for its 

 spherulitic structure. 



The dyke-like offshoots which break the even western boundary of the Mid 

 and Wester Hills must be transgressive in their relations. The middle one, 

 as exposed in a quarry south of some targets, cuts right across the bedding of 

 the vertical Silurian, and then runs along parallel to the bedding. 



The only evidence suggesting the presence of lavas is the rather 

 stratiform appearance of the hills. This is most probably due to 

 the intrusion of the mass sheet by sheet. 



The steeper slopes of the Mid and Wester Hills are covered 

 by scree, while the North Hill is much overgrown with grass : 

 continuous zoning of the intrusions is, therefore, fraught witli 

 considerable difficulty. 



The North Hill has a particularly stratiform appearance. 

 The lowest portion consists of por phyri tic sanidine- trachyte, 

 with a spongy aggregate of secondary limonite which recalls the 

 habit of riebeckite. This rock is found next to the Old Red 

 Sandstone in many portions of the complex. Above this, pink 

 non-porphyritic sanidine-trachj^te with much quartz is 

 sometimes found ; it is composed of sanidine-laths, abundant 

 mossy limonite, probably after riebeckite, and a fair proportion of 

 interstitial quartz. Specimens from below the prominent crags 

 on the north-west side of the hill, and from a corresponding 

 point farther east, show quartz -phenocrysts quite clearly. Under 

 the microscope the quartz-phenocrysts exhibit sharp idiomorphic 



