part -i] MATEEIAL OF BUNTER PEBELE-BEDS. . 381 



zone-mineral of this formation. The grains are often rough and 

 angular with frayed and toothed edges, averaging about 0"35 mm.. 

 in greatest diameter. Nearly spherical grains occur. Its colour 

 varies from pale j^ellow, through amber, to reddish -brown. 

 Numerous grains are quite fresh, though some in a more or less- 

 decomposed state are present. Among frequent inclusions are 

 opaque black particles, also quartz. Some grains appear to be pure. 



The micas. — White mica is plentiful in most places, especially 

 in the eastern outcrop. The Hakes are mostly parallel to (001), 

 and exhibit usually a low grey interference-colour between crossed 

 nicols. Strain-shadows are frequent. The average diameter is 

 about 0-53 mm. 



A pale-green variety is not uncommon which, in contradistinction 

 to the above white mica, mostfy remains dark between crossed 

 nicols. 



Biotite, so far as I can determine, is rare. 



Felspar is usually found in a very decomposed and clouded 

 state. Orthoclase, which is probably plentif id. may be recognized : 

 strain -shadows are frequent. Only one or two grains of plagioclase 

 have been cleliniteh' distinguished. Microcline is not uncommon. 



IV. COXCLUSIOXS AS TO THE PEOBABLE DlEECTIO^ OF THE 

 SeDIMEXT-BEAEIXG CuEREXTS, AXD as TO THE SOUECES 

 OF THE MiXEEAL (xEAIlS'S. 



From the distribution of the aforementioned heavy minerals, 

 together with such evidence as the dip of the false bedding and the 

 variation in thickness of the Pebble-Beds, the direction of the chief 

 sediment-bearing currents may be deduced. 



From the kind of mineral and the character of certain pebbles, we 

 can reasonably infer the source of origin of much of the material. 



The accompanying map and table (p. 332) will help to 

 demonstrate the distribution of the heav}^ minerals. 



Along the western outcrop) the highest percentage of heavy 

 minerals, with a specific gravity of 2*77 and upwards, is at Gorse- 

 thorpe. From Grorsethorpe a diminution in the percentage is 

 continued northwards, hy way of Carburton, to Bawtry. South of 

 G-orsethorpe the fall in the percentage of heavy minerals is continued,. 

 by way of Python Hill, to Nottingham. 



Along the eastern outcrop we find the, highest percentage at 

 Ollerton and Farnsfield. North of Ollerton to Retford there is 

 a considerable decrease ; while from Farnsfield to Calverton, in the 

 south, there is a marked diminution in the percentage of heavy 

 minerals. 



The current -bedding. — At Retford and Ollerton i the pre- 

 vailing dip of the cross-bedding is in an easterly direction. On 



^ 'The Geology of the Countiy aroiiiid Olleitou ' Mem, Geol. Siirv. 1911, 

 p. 23. 



