Vol. 60. "1 OF THE EAST-CENTRAL HIGHLANDS. 420 



to be filled up by the folded higher beds. This happens along 

 almost the whole length of the south-eastern side of the Ben-y-Ghloe 

 Mountains, which rise from comparatively low ground to a height 

 of more than 3600 feet above sea-level. The gap to be filled up is 

 exceptionally large, and in consequence the largest known outcrop 

 of the Calc-Flintas occurs here, and we see in addition a small 

 number of thin infolds of the altered dark shale originally above the 

 Calc-Flintas. But over the whole of the rest of the country, the 

 rock on the reverse side of the flintas to the Main Limestone is 

 never seen ; and the flintas must in all other cases be either the 

 highest or lowest rocks in this part of the South-Eastern Highlands, 

 and the evidence is conclusive that they are the highest. 



This view, that there is a descending succession from the Main 

 Limestone to the Quartzite, is greatly strengthened by the frequency 

 with which a hiatus occurs at the margin of the Main Limestone, a 

 good example of which is shown on the map (fig. 4, p. 422) in the 

 corrie opposite Newbiggin. Sometimes the whole of the Dark Schist 

 and the Little Limestone are missing, sometimes portions only; but 

 as the investigation proceeds, it will be seen that the line of erosion 

 at the base of the Main Limestone is not by itself sufficient to 

 explain all the phenomena met with. 



{g) Meaning of the Patches of Dark Schist and Proof that 

 the Sequence is incomplete in the Glen-Tilt Area. 



The meaning of the patches of Dark Schist in the Gilbert's- 

 Bridge and Banvie-Burn sections can now be investigated. 1 Below 

 Gilbert's Bridge, close to Crombie Burn, one of these patches inter- 

 vening between the limestone and the Moine Gneiss is a kyanite- 

 garnet-stauroiite-schist, obviously well above the bottom of the 

 dark shale, but almost certainly a little below the top. There are 

 several other smal] patches below Gilbert's Bridge, and these appear 

 to be approximately at the same horizon. Nearer Gilbert's Bridge 

 a film of tough schist occurs between the Limestone and the Pink 

 Felspathic Bock. This (10,549) is a highly-micaceous rock, built 

 up of alternating films of quartzose and micaceous material ; the 

 latter consist largely of white mica and chlorite, often enveloping 

 large cracked and decomposed garnets. Iron-ore is abundant, both 

 in good-sized grains and as fine dust, and it is often embedded in a 

 clear, almost glassy material, which is known to be plagioclase 

 although it here shows no striatum. There can be little doubt 

 that this is a siliceous modification of the Felspar-Rock, and it 

 illustrates a difficulty that occurs repeatedly. As the area is 

 approached where the more sandy material, now forming the Moine 

 Gneisses, was deposited, the Dark Schist tends to become slightly 

 more siliceous, and differs slightly in appearance from the rocks of 

 the type-area. If the siliceous material increases beyond a certain 

 point, the zone can, of course, be no longer identified. So far as is 



1 See Map, PI. XXXITI. 



