PEOCEEDINGS 



OF THE 



Camtrritrge IJHtosojjIjkal Sbrietg. 



January 28, 1884. 

 Mr Glaisher, President, in the chair. 



The following communications were made to the Society: 



(1) On the Microscopic structure of a Boulder from the Gam- 

 bridge Greensand found at Ashwell, Herts. By Prof. T. G. 

 Bonney, D.Sc, F.R.S, F.G.S. 



This boulder was found by H. G. Fordham, Esq., F.G.S., by 

 whom a fragment was sent to the author for examination, with the 

 following remarks. 



"The boulder measures 12 x 9 J x 5£ inches, and is therefore 

 amongst the largest at present known from this bed. It is some- 

 what triangular in general form, one surface being nearly flat, and 

 it is very much rounded and worn. On the weathered surface 

 dark purple wavy lines appear, generally of the thickness of a sheet 

 of writing paper, but sometimes a quarter, or even half an inch 

 thick, alternating with lighter and thicker bands. Where broken 

 the rock is more uniform in colour, the bands varying in shades of 

 purple. Occasionally, where much weathered, the lighter bands 

 show a tendency to columnar structure, developed perpendicularly 

 to the planes of banding. The material is very hard, and not 

 easily broken. The surface of the boulder is worn and smoothed, 

 and in some parts may almost be said to be polished. Here and 

 there the softer material of the light-coloured bands has been worn 

 into small cavities or depressions, and in other places the lines of 

 banding are brought into strong relief by a more uniform wearing 

 away of the softer bands. 



VOL. V. PT. II. 5 



