292 DR. A. GILLIGAX ON THE PETROGRAPHY OF [vol. lxxv, 



Fig. 2. Pebble of granite from the Middle Grits, Silsden. The felspar is 

 oligoclase, very fresh, often enclosing quartz. A little hornblende 

 is present. X about 25 diameters. (See p. 255.) 



3. Pebble of granite from the Middle Grits, Silsden. Quartz (opalescent), 



fresh felspar, andmuscovite are distinguishable in the hand-specimen. 

 The felspar shows curved lamellse, and encloses blebs of quartz. 

 X about 25 diameters. (See p. 255.) 



4. Pebble of granite from the Middle Grits, Silsden. This appears to be 



a fine-grained felspathic gneiss, but under the microscope shows no 

 trace of having been crushed. The rock is a protoclastic granite, 

 singularly fresh. X about 25 diameters. (See p. 256.) 



5. Pebble of quartz which encloses a fragment of mica-schist, from the 



Middle Grits, Silsden. The quartz bears evidence of shearing. The 

 mica- schist is of the usual type. Vermicular chlorite occurs in the 

 quartz. X about 20 diameters. (See p. 256.) 



6. Pebble of mica-schist, very much contorted, from the Middle Grits, 



Silsden. Garnets with good crystal outline are present, but fre- 

 quently part of the garnet has been replaced by some other isotropic 

 mineral which has not been determined. X about 20 diameters. 

 (See p. 256.) 



Plate XVIII. 



Pig. 1. Pebble of black lustrous mica-schist from the Middle Grits, Silsden. 

 Magnetite has separated out prior to the final folding, as it has con- 

 formed so well with the lines of folding. Quartz is fairly abundant, 

 while no fresh felspar can be detected. X about 25 diameters. 

 (See p. 257.) 



2. Pebble of black lustrous mica-schist from the Middle Grits, Silsden. 



Quartz is fairly abundant. Muscovite and biotite are the principal 

 constituents, together with a white chlorite. Much black diist 

 occurs in the felspars. This is the pebble identified as the Black 

 Schist of the Blair Athol-a-Nain district. X about 25 diameters. 

 (See p. 257.) 



3. Pebble of greenish slaty rock, with pronounced folding, from the 



Middle Grits, Silsden. The most noticeable feature is the beautiful 

 microfoliation with a tendency to strain-ship cleavage. X about 

 25 diameters. (See p. 257.) 



4. Pebble of black chert from the Kinderscout Grit, The Strid, Bolton 



Abbey. A fine-grained chert, deeply stained with iron-oxide and in 

 part isotropic. One edge shows typical oolitic structure. X about 

 25 diameters. (See p. 257.) 



5. Pebble of grey chert from the Plumpton Grit, Knaresborough. The 



oolitic grains have their borders defined by a number of small 

 rounded bodies, which do not show any organic structure. Numbers 

 of these bodies occur scattered through the rock apart from the 

 oolitic grains. The section bears a striking resemblance to a pebble 

 from the Torridon Sandstone, described in the North- West Highland 

 Memoir, p. 280 & pi. 1, fig. 1. X about 25 diameters. (See p. 257.) 



6. Pebble of black chert from the Rough Rock, Cragg-Hill Quarry, 



Horsforth. By ordinary light outlines of organisms can be seen, 

 probably sponge-spicules. In polarized light no trace of these 

 organisms can be seen, and the rock appears to ba made up of small 

 grains of quartz embedded in a fine-grained siliceous matrix, x about 

 25 diameters. (See p. 258.) 





