Reports and Proceedings — Geological Societtj of London. 23-> 



is given of a series of sections lying along a belt of 40 miles in 

 length, extending nearly from Blair Atholl to the east of Balmoral, 

 in Aberdeenshire. The first and most important of these occurs- 

 about Gilbert's Bridge (in Glen Tilt), where the parallel banded 

 Moine gneisses can be traced passing slowly into the honestones, in 

 which parallel banding is equally well shown. This is a well-known 

 horizon in the Central Highland sequence, lying next the white 

 edge of the Highland Quartzite, forming, in fact, its orginal flaggy 

 margin. These parallel banded rocks are in many cases succeeded 

 directly by a very impure phase of the Main or Blair Atholl Lime- 

 stone ; but in places patches of other material intervene, of which 

 the most important is a dark schist: this suggests a small hiatus 

 at the margin of the Limestone, and a photograph was exhibited to 

 show this hiatus. The conclusions drawn from this section are 

 supported by the section seen below Gilbert's Bridge, and a somewhat 

 similar one in the Banvie Burn, north of Blair Castle. As before, 

 there is clearly a small hiatus at the base of the Limestone. 



In order to ascertain the meaning and extent of this break in the 

 sequence, an account is next given of the complete succession near 

 Braemar, and it is then seen that at Gilbert's Bridge the Little 

 Limestone and part, or at times the whole, of the Dark Schist i& 

 missing. The hiatus always tends to occur as an area is approached 

 where the material forming the Moine gneiss thickens, and was 

 originally of a rather coarser or more sandy nature. 



Where, however, the section is complete, it is seen that the 

 material of the Moine gneisses is the flaggy margin or top of the 

 Central Highland Quartzite ; it is succeeded by the Little Lime- 

 stone, above which is the Dark Schist, and then the Main or Blair 

 Atholl Limestone. 



Other sections along the line of change are described, showing the 

 varying phases of the honestones, and in two instances their passage 

 into Moine gneiss. There is a constantly varying hiatus at the base 

 of the Main Limestone, but in the whole 40 miles this never exceeds 

 the omission of the entirety of the Black Schist and the Little Lime- 

 stone (of no great original thickness). This break in the sequence 

 is of small importance, and, as already stated, often disappears as 

 the material from which the Moine gneisses were formed became 

 thinner and finer, or more of the nature of a mud. 



Evidence is then given to show that the honestones tended to 

 become more sandy and to thicken south-eastward again, or in 

 the opposite dii'ection to that in which the Moine gneisses come on. 

 From this the author concludes that the parallel banded material 

 was deposited in a series of fans ; in the larger fans we have the 

 material of the Moine gneisses ; in the smaller that of the honestones. 

 Both are simply the flaggy top of the sandstone now forming the 

 Central Highland Quartzite, and are in fact a passage rock on its 

 margin. Anything like an unconformity between the two is^ 

 obviously impossible. y 



