part 1] THE MILLSTONE UlilT OF \ I >B k 8B 1 i: E. 257 



as mica-schists and quartz-schists. The former show contortions 

 with minute foliations and strain-slip cleavage. Among these 

 pebbles is found one which may provide a clue to some of the 

 problems concerning the source whence the material of the grit 

 was derived (see PL X VIII, fig. 2). It has been identified both 

 by Prof. T. (1. Bonney and Mr. (J. Barrow as coming from the 

 Blair Athol district. Mr. Barrow writes: — 



' This is the " Black Schist " associated with the Blair Athol-a-Nain Highland 

 Limestone. Along a great belt of country the lower part of this rock is 

 known as the " Felspar-Kock " of Glen Callater, as that was the place where 

 it w&e first found. It succeeds a rock especially rich in peculiar forms of 

 chlorite, and there is much of this chlorite still in the lower part of the bed 

 altering to the Felspar-Rock, but less as we ascend. This rock is from the 

 lower part of the " Felspar-Rock " ; it contains the felspar (pale-grey under 

 crossed nicols) first determined by Sir Jethro Teall, and in this felspar is the 

 typical dark dust. In addition the pale, almost white chlorite is present 

 in considerable quantity, and the characters of the rock as a whole make it 

 quite unmistakable.' 



With this schist must undoubtedly be placed, as allied in origin. 

 some of the others, as they contain the pale chlorite, and present 

 a general family resemblance. 



Some of the schists are garnetiferous, and thus help to explain 

 the common occurrence of garnets in the grit-beds; while tourmaline, 

 though not previously discovered in the sections prepared of these 

 rocks, Is nevertheless Found in considerable quantity in the heavy- 

 mineral separations. Zircons are plentiful, while rutile is also 

 fairly common in some of them. 



A few pebbles of slightly-altered sedimentary rocks have also 

 been found. 



{(/) Pebbles of Sedimentary Hocks. 



In the beds of coarse grits at the base and top of the Millstone 

 Grit Series, and in the coarser bands of the Middle Grits, occur 

 numerous pebbles of chert. These are usually black and therefore 

 easily detected, but others are buff, brown, or yellow. Generally, 

 they present the ordinary characters of such rocks when seen under 

 the microscope Many of these chert-pebbles from the Kinderscout 

 Grit of Upper Wharfedale and Airedale have been sliced and 

 compared with specimens of chert from the Carboniferous Lime- 

 stone Series; and it has been found that, while organic structures 

 are usually present in those from the Carboniferous Limestone, 

 only a few from the grits show any traces of organisms and then 

 of quite a different nature. 



One of the chert-pebbles (see PI. XVI II, fig. I) found in the 

 Kinderscout Grit of The Strid, Bolton Abbey, shows beautifully- 

 developed oolitic structure. From the Plumpton Grit at Grimbald's 

 Crag, Knaresbbrough (exposed in the river-bed I, was obtained a very 

 curious grey pebble (see PI. XVIII, fig. 5), which, in tin' hand- 

 specimen, shows a number of circular pits suggesting oolitic 

 structure, in many parts the pebble is stained with haematite. 



