274 DE. A. GILLIGAN OX THE PETKOGEAPHY OE [vol. IxXV, 



Heavy ^Minerals in Pebbles found in the Millstone Grit. 



a = abundant. vc = very common. 

 c= common. f= frequent. 

 r=rare. vr=very rare. 



=« ,2 



~ '43 ! § I 2 3 5 



s 



Granite (3 in Coll.) 



(5 in Coll.) 



(10 in Coll.) 



(11 in Coll.) 



Schist (16 in Coll.) 

 (18. in Coll.) 





r 



f 



c 



a 



f 



c 





r 



c 



f 



vc 



r 



a 







vc 



f 



a 





vc 







f 



c 



vc 





f 







vc 



f 



vc 



vr 



vc 



a 





c 



c, 



vc 





vc 



c 



Brown 



(Blue r) 



a 



■a i 



P< o 



vr 

 r ! f 



We may now sum up the evidence of the litholo- 

 gical characters. 



Pebbles of the following types have been found and examined : — 

 Gneisses, schists (of various types), granites, pegmatites, quartz- 

 and felspar-porphyries, quartz, felspar, quartzites, cherts, sand- 

 stones, and mudstones. 



The large quartz-pebbles, which are the most plentiful of all the 

 pebbles in the grit, are of such shape and microscopic structure as 

 to make it certain that they have been derived from rocks which 

 have been subjected to intense shearing, while strain-shadows are 

 invariably present. The smaller pebbles and grains of quartz are 

 commonly of a bluish or opalescent tint, the colour being due 

 to inclusions of various types — regular, irregular, and acicular ; 

 while liquid inclusions with movable bubbles are exceedingly 

 common. 



The dominant felspar of all the beds is microcline. This occurs 

 in large flesh-coloured pebbles, commonly about | inch in diameter, 

 and is always quite fresh and unaltered even when it has been ex- 

 posed on the face of a quarry or building for a considerable period. 

 It occurs plentifully, too, as smaller grains in the mass of the grit 

 or shale, where it is also quite unaltered. Other types of felspar 

 found are microcline-microperthite. oligoclase, and a little ortho- 

 clase. The oligoclase and orthoclase are frequently altered, and 

 I am of the opinion that the greater part of the altered felspar 

 found in the grit has been derived from this source. 



Pebbles and grains derived from sedimentary rocks are plentiful, 

 by far the most abundant being chert (of various colours). In 

 some of these, as already noted, traces of organisms and well- 

 defined oolitic structure are found (see PI. XVIII, figs. 4-0). 

 These must represent the solution and decay of limestone-beds. 



Sandstone pebbles also occur, sometimes of large size ; while 

 altered mudstone and mudstones with organisms are included in 

 the series. 



