72 ME. A. GEIKIE ON THE A.GE OE THE 



ration turns north-westward, and can be traced for some 200 yards 

 along the steep declivity and the rocky shore until, owing to the 

 trend of the coast, it strikes into Loch Slapin. This is the locality 

 to which I haTO already referred as so deceptive in regard to the 

 real relations of the two series of limestones. The fossiliferous 

 Lias beds, dipping south-westwards at an angle of about 20°, con- 

 sist of well-bedded limestone and calcareous shale, some of the 

 layers being charged with Gryphcea. Immediately underneath 

 them, and without the intervention of any conglomerate, come the 

 Silurian limestones, the dip of which, though not very distinct, 

 seems to be parallel with that of the younger strata. The two 

 groups are here, as it were, welded together at the juncture. Yet 

 all along the slope above, the older limestone with its abundant 

 cherts stands on end, with a north-west strike, which is maintained 

 as it runs out to sea on the one hand, and away inland on the other. 

 Another outcrop of the basement-beds of the Lias may be traced 

 between Heast and Boreraig on Loch Eishort. At Heast a thick 

 group of conglomerates and pebbly sandstones underlies the lowest 

 of the Lias limestones. These detrital beds, with a band of lime- 

 stone-conglomerate in the middle of them, rest on the red 

 sandstone along the base of the escarpment on the east side of Beinn 

 a Chairn. But as they are followed south-westwards they disappear, 

 and the limestone, in some places full of fragments of quartzite and 

 black chert, lies directly upon the red sandstone and quartzite 



(fig. 1). 



(3) The Granophyre bosses. — Though it is no part of my purpose 

 to describe the contact-metamorphism which has given to the lime- 

 stone of Strath its place in geological literature, I may refer to the 

 relation of the limestone to the Tertiary eruptive rocks of the 

 district, and to the alteration which these have superinduced. In 

 my early paper on the geology of Strath, I pointed out that the 

 *' syenites " of this district were easily divisible into two groups, one 

 of which embraces disruptive bosses that ascend with steep sides 

 through the surrounding rocks ; while the other includes overlying 

 sheets which spread over the strata without violently disturbing 

 them. The separation of the Silurian from the Lias limestone now 

 enables us further to perceive that the bosses are confined to the 

 Silurian area, and the overlying masses to the territory of the Lias. 

 The latter have produced only a trifling alteration of the rocks. 

 Except therefore along the flanks of the Eed Hills, where, in some 

 places, portions of the Secondary strata have been thrust up and 

 invaded by the large eruptive masses, the Liassic beds are not 

 seriously altered. The metamorphism for which Strath has so long 

 been known turns out to be confined to the Silurian limestone. On 

 the whole, it is restricted to the near neighbourhood of the bosses 

 of granophyre, and consists in the usual marmarosis or assumption 

 of a saccharoid crystalline texture. The white limestones become 

 granular statuary marbles, and their bedding partially or wholly 

 disappears. The darker beds retain their dull leaden-grey hues, but 

 become coarsely crystalline and still expose, on weathered surfaces, 

 their abundant rugged prominences of black chert. 



