744 Geological Society : — 



broken, are found to be perfectly fresh, the lustre of the cleavage- 

 faces being most remarkable. Blebs of quartz are frequently 

 present in these felspars. In many of the rock-sections grains of 

 microcline and oligoclase, quite fresh and unaltered, are common. 

 Fragments showing the intergrowth of blue or opalescent quartz 

 and microcline are fairly abundant. 



Chert-pebbles are plentiful in the coarse beds at the base of the 

 series ; they are also sporadically distributed throughout the upper 

 beds, and in some of these oolitic structure has been observed. One 

 pebble of silicified oolite shows a microscopic structure strongly 

 resembling a structure found in the Torridon Sandstone. A few 

 fragments containing microscopic organisms have also been obtained. 



Mica is not plentiful in the coarser beds, but increases in amount 

 with decrease in grade of the material. 



From the Middle Grits of Airedale a remarkable assemblage of 

 pebbles has been obtained, including the following tj r pes : — gneisses, 

 granites, schists, quartz- and felspar-porphyries, quartzites, grits, 

 sandstones, and mudstones. One of these pebbles has been recog- 

 nized as the black schist associated with the Blair Athol-a-Nain 

 Limestone of Scotland. Another pebble is doubtfully referred to 

 the rhomb-porphyry of the Christiania region. 



The results of the investigations into the heavy mineral contents 

 may be summarized as follows, dividing for this purpose the 

 Millstone-Grit Series into three more or less well-defined groups : — 



(a) Lower Division — Base of the Ingleborough Grit to the base of the Leathley 



Sandstone. 



(b) Middle Division — Leathley Sandstone to the base of the Flags below the 



Rough Bock. 



(c) Upper Division — Flags and Bough Bock. 



The minerals are tabulated in decreasing order of relative 

 -abundance : — 



(a) Coarse beds contain garnet, ilmenite and leucoxene, zircon, tourmaline, 

 rutile, monazite and magnetite. 

 Fine beds contain zircon, rutile, garnet, and tourmaline. 

 . (6) Coarse beds contain zircon, rutile, garnet, tourmaline, ilmenite and leuco- 

 xene, magnetite and monazite. 

 Fine beds contain zircon, rutile, tourmaline, and garnet. Some of the 

 separations from the shales of Otley Chevin were almost entirely 

 zircons, only a few grains of other minerals being present. 

 ■(c) Coarse beds contain garnet, ilmenite and leucoxene, zircon, rutile, tour- 

 maline, monazite and magnetite. 

 The Flags at the base of the Bough Bock contain zircon, rutile, garnets, 



and tourmaline. 

 The monazite has been determined by spectroscopical and chemical tests. 



In view of the similar work which is being done among the 

 younger sedimentary rocks, it is important to record that, although 

 the author has not yet discovered staurolite in the Millstone Grit, 

 he has found it to be common in some of the sandstones near the 

 top of the Coal Measures in Yorkshire : — namely Ackworth Rock. 

 Pontefract Rock, and the Red Rock of Rotherham, and also in 

 basement Permian at Conisborougrh. 



