Notes and Comments. 53 



DISCUSSION. 



Professor E. J. Garwood quite agreed that the form, 

 generally known as ' Phillipsastrcea ' radiata, which occurs 

 in the Botany Beds in Yorkshire and also in the Fell Top 

 Limestone of Northumberland, was a distinct form apparently 

 limited to a high horizon in the Yorkshire beds, and he had 

 himself used it as a zonal index for this horizon. He pointed 

 out that at Botany, the beds still contain abundant examples 

 of Dibunophyllids and other well-known marine Lower Car- 

 boniferous forms, although they occur some 200 feet above 

 the base of the Millstone Grit Series of the Geological Survey 

 maps. It was obvious, therefore, that, however useful it 

 might be for economic purposes to represent the arenaceous 

 occurrences by a special colour, this sandy episode entered 

 in different districts at different periods, and could not be 

 used as a definite stratigraphical horizon dividing the Lower 

 and Upper Carboniferous rocks. 



FACETTED PEBBLES FROM LANCASHIRE. 



At the same meeting, Mr. J. W. Jackson exhibited a number 

 of facetted pebbles from Pendleton (Lancashire), and stated 

 that nearly 200 of these had been collected during the last 

 six months from near the top of a section of current -bedded 

 and faulted Glacial Sand and Gravel at an altitude of about 

 200 feet O.D. The pebbles occur in situ some 2 or 3 feet 

 below the capping of darker subsoil, which contains cores 

 and flakes of flint, including pigmies. They consist of slate, 

 granites (Eskdale and Shap), Ennerdale granophyre, Borrow- 

 dale volcanic tuffs, porphyries, quartzites, Millstone Grit, 

 sandstones, Chalk flints, Carboniferous chert, and other rocks. 

 The largest facetted pebble measures njx8£ inches, and is 

 7 inches high ; the smallest is only half an inch in diameter. 

 The facets are generally concave, grooved, or fluted. They 

 vary in number : some stones have one facet only, others two 

 or more. One stone with a flat top shows five incipient facets. 

 On some, the grooving is of the nature of parallel series of 

 elongated pits. 



DIFFERENTIATION. 



Differentiation, according to varying hardness and com- 

 position, is well displayed on the granites, porphyries, grits, 

 etc., where the weaker constituents have been strongly eroded, 

 leaving the stones with an irregularly pitted surface. The 

 production of facets by splitting along joint-planes is seen 

 on some examples of sandstone ; but the facet thus formed 

 has been modified by wind-action. A few pebbles occurred 

 in the sand completely inverted, and some show distinct 

 facetting on both sides. Of examples orientated in situ, the 

 facets faced north-westwards, westwards, and south-west- 



1917 Feb. 1. 



