Vol. 56.] CONSTITUENTS OF THE CRAG. 743 



Norwich Crag (Icenian). 



More than 3 lbs. of Norwich Crag was treated with heavy fluids 

 in order to concentrate the minerals. Large fractions were obtained, 

 and these have been thoroughly examined with a view to detecting 

 possible differences between the inorganic constituents of the Red 

 and Norwich Crags. 



All the minerals stated as occurring in the Red Crag have been 

 found in the Norwich Crag ; the latter contains more mica, the 

 grains are not so heavily stained with iron-oxide, and a fine muddy 

 material coats the sand-grains. 



Lithologically the Red and Norwich Crags must be regarded as 

 forming one series, deposited under somewhat similar physical 

 conditions. 



Chillesford Sand. 



Specimens were examined from two localities : the pit behind 

 Chillesford Church, and the brickfield at Aldeburgh, where the sand 

 is seen underlying Chillesford Clay. The minerals from these two 

 localities are exactly the same, the only difference being that the 

 grains are somewhat larger at Aldeburgh. 



The sand was fractionated with borotungstate of cadmium, and 

 the various separations were examined separately. 



The following minerals occur : — Zircon, rutile, garnet, andalusite, 

 ilmenite, leucoxene, tourmaline (green, blue, and yellow), biotite, 

 muscovite, a green mica, plagioclase-felspars, quartz, and flint. 



In striking contrast with the Crag sands, it will be noted that no 

 glauconite was found. 



Mica is very abundant, especially muscovite ; and grains of flint 

 are rare. Ferro-magnesian minerals except biotite are absent. 



Nearly all the minerals enumerated in the foregoing pages are 

 stable in composition, and capable of transport for long distances 

 without decomposition. It would be a fair inference to assume 

 that they have come from a distant source, and probably had their 

 origin in rocks which have undergone extensive metamorphism. 

 Whether they have been directly derived from such rocks would be 

 hard to prove, but the frequent inclusions of zircon, rutile, and 

 other secondary minerals in mica and other substances favours this 

 view. 



DrscussioN. 



Mr. H. W. Burrows agreed with the Author's general contention 

 that the Upper Red Crags show a succession of oldest to newest 

 from south to north. This, he believed, could be demonstrated in 

 detail : the pits at Butley being cited as an illustration. There seemed 

 to him a peculiar fitness in the fact that the Author, who had 

 endeavoured to demolish the seven zones established by Prestwich 

 for the Coralline Crag, should now replace them with an equivalent 

 number for the Upper Crags. According to the Author, the whole of 

 the Coralline Crag could be included under one phase — Gedgravian ; 

 while it was necessary to define as many as seven distinct zones for 



