part 4] THE MILLSTONE URIT or YORKSHIRE. 291 



EXPLANATION OF PLATES XV-XVIII. 



Plate XV. 



Fig. 1. Quartz-pebble from Kinderscout Grit, The Strid. Bolton Abbey. The 

 specimen is a good example of mylonitization, with evidence of 

 shearing in the elongated patches of quartz, well seen in the photo- 

 graph. X about 20 diameters. (See p. 262.) 



2. Pebble of blue quartz from the Middle Grits near Ripon. The acicular 



inclusions are well shown, as are also the liquid inclusions which 

 contain movable bubbles. X about 110 diameters. (See p. 254.) 



3. Pebble of black quartz from the Rough Rock. Cragg-Hill Quarry, 



Horsforth. A very common type, the strain- shadows and ' mortar- 

 Btructure ' resulting from pressure prior to inclusion in the grit. 

 X about 20 diameters. 



4. Pebble of microcline from the Rough Rock. Cragg-Hill Quarry, Hors- 



forth. X about 20 diameters. (See p. 254.) 



5. Pebble of felspar from the Rough Rock, Clayton- Wood Quarry, Hors- 



forth. This also is a common type, consisting of a microperthitic 

 intergrowth of microcline and oligoclase. X about 20 diameters. 

 (See p. 254.) 

 0. Kinderscout Grit, Embsay Moor, near Bolton Abbey. The freshness 

 and rounding of the microcline is noticeable, and the term 'arkose' 

 is correctly applied to such a rock. 



Plate XVI. 



Fig. 1. Rough Rock from borehole, at a depth of 125 feet, Meanwood, Leeds. 

 Fresh plagioclase felspar is uncommon. x about 20 diameters. 

 (See p. 262.) 



2. Pebble of quartz-porphyry, from the Middle Grits, Silsden. The 



ground-mass shows traces of devitrification, and the quartz is often 

 much corroded. X about 20 diameters. (See p. 256.) 



3. Pebble of quartz-porphyry from the Middle Grits. Silsden. This 



differs in type from that illustrated in fig. 2, in that it contain- 

 much decomposed and corroded orthoclase. X about 20 diameters. 

 (See p. 256.) 



4. Pebble of porphyry from the Middle Grits, Silsden. In the ground- 



mass quartz and felspar occur in micrographic intergrowth of an 

 extremely minute character. The felspars are chiefly albite and 

 oligoclase. with some orthoclase. X about 25 diameters. (See 

 p. 256.) 



5. Pebble of pegmatite, from the Kinderscout Grit, Skipton. Pegmatite 



consisting of quartz intimately intergrown with microcline (which is 

 very fresh) and microperthite. X about 20 diameters. (See p. 255.) 



6. Pebble of pegmatite from the Rough Rock, Cragg-Hill Quarry, Hors- 



forth. In the hand-specimen the quartz appears opalescent, and 

 shows under the microscope inclusions similar to those seen in 

 PI. XV, fig. 2. The dark areas with good crystal outline are caused 

 by the removal of felspars during the slicing process. X about 

 20 diameters. (See p. 255.) 



Plate XVII. 



Fig. 1. Pebble of granite from the Middle lints. Silsden. This is the largest 

 pebble yet obtained from the Millstone Grit, measrtring 10 by 8 by 

 1 inches. It i- of a general pink colour, with large white and grey 

 patches of altered felspars. Hornblende and biotite both much 

 altered, with separation of iron-oxides. The biotite has numerous 

 small zircons enclosed. x about 2-"> diameters. (See p. 255.) 



