part I ] 



THE MILLSTONE (.KIT OF YORKSHIRE. 



285 



formations ; in fact, they represent the screes formed on the slopes 

 of the old land-surface upon which the Torridonian rocks were 

 laid down. The conditions which obtained during the time, and 

 the area of deposition of the basement-beds, of the Millstone Grit, 

 were of course quite different. 



Further, complete agreement even in the general constituents of 

 the two formations cannot be expected, since, as I have already 

 mentioned, some part of the allothigenons material of the Millstone 

 Grit must necessarily have been derived from rocks of later age 

 than the Torridonian, and situated in areas which were unlikely or 

 impossible sources of the material that makes up the Torridonian. 



The following comparison will illustrate the resemblances and 

 differences : — 



Torridonian. 



( a ) Presence of blue or opalescent 

 quartz with acicular, irregular, and 

 regular inclusions. 



(b) The dominant felspars are 

 microcline, microcline-microperthite, 

 oligoclase. and orthoclase. The cha- 

 racteristic felspar of the arkose> i- 

 microcline and microcline-perthite. 

 quite fresh and unaltered. 



(c) Cherts of various colours show- 

 ing cryptocrystalline or microcrys- 

 talline structure. No definite or- 

 ganisms have yet been found. 



(d) Silicified oolite, in which the 

 oolitic grains are defined by small 

 spherical bodies and pseudomorphs in 

 the form of rhombs showing their 

 derivation from calcareous rock>. 



(e) Quartzite-pebbles. 



(/) Pebbles of vein-quartz, which 

 -how evidence of shearing, are the 

 most abundant. 



(g) In the larger pebbles are found 

 felsites. gneiss, mica-schist. 



(Ii ) Clastic micas (both brown and 

 white) arc present in the finer-grained 

 beds. 



(0 The heavy minerals include 

 magnetite, ilmenite, sphenc. garnet, 

 tourmaline, zircon, and rutile ; Dr, T. 

 O.Bosworth (Rep. Brit. Assoc. Dundee,. 

 1912. p. -174) points out the great 

 abundance of garnet and zircon. 



Millstone Grit. 



(a) Similar pebbles abundant in 

 the grit. 



(b) Microcline. microcline-micro- 

 perthite. oligoclase, and orthoclase 

 occur, but microcline and microcline- 

 perthite are by far the most abun- 

 dant in the coarse grits and quite 

 fresh to the outside. 



(c) Cherts of various colours show- 

 ing similar structure are common in 

 the coarse beds. Some ghosts of 

 organisms are found in a few of the 

 chert-pebbles. 



(d) Exactly similar pebbles have 

 been found, in which the small round 

 bodies are about the same size as in 

 the pebble figured and described in 

 the N.W. Highland Memoir, p. 280 & 

 pi. 1, fig. 1. See PI. XVIII, fig. 5. 



(e) Quartzite-pebbles. 



(/) This is also the case with the 

 Millstone Grit. See figures. 



(g) Pebbles of similar types are 

 here figured from the Grit. 



(h) Clastic micas (both brown and 

 white) are present in the finer-grained 

 beds : sometimes the white mica 

 makes up beds from 2 to 3 feet thick 

 with only a small admixture of quartz, 

 felspar, and oxide of iron (e. g., below 

 the Rough Rock, Keighley Moor). 



(i) The heavy minerals include 

 ilmenite and leucoxene. garnet, zir- 

 con, tourmaline, rutile. and monazite. 

 Of these by far the most abundant 

 is garnet; while zircon is very com- 

 mon in all the beds, but especially 

 in the Middle Grits ami Rough Rock. 



