BY W. N. BENSON. 151 



fresh, but somekaolinisation has taken place, spreading outwards 

 in bands from the cracks separating the fragments of which the 

 rock is composed. The reddish colour of the rock is due to the 

 oxidation of scattered grains of pyrites. 



Magnetite enters into these rocks in a variety of ways, one of 

 the most remarkable being that seen in the nodular magnetite- 

 keratophyres(1096) [Plate xxvii., fig. 3]. The rock is divided up 

 into roughly polygonal masses about four millimetres in diameter. 

 Each polygon consists of an outer rim, rich in finely crystalline 

 magnetite in a network of albite microlites. There follows 

 within a zone of varying width, consisting of a finely granular 

 mosaic or lathy felt of albite, which sometimes contains large, 

 clear phenocrysts of the same mineral. There is usually no 

 general circumferential or centripetal arrangement of the felspar 

 laths within a nodule, but frequently a general trachytic structure 

 continues without interruption throughout the whole nodule, 

 and may be parallel or inclined to the trachytic arrangement 

 in adjacent nodules. Besides the albite, a little chlorite may 

 occur in this zone. Within this is a narrow passage-zone of 

 felspar laths, sometimes more or less kaolinised, and containing 

 rather abundant dusty magnetite. The inner part of the zone 

 may be coloured yellow by the abundance of the oxidised chlorite. 

 The central portion is of normal magnetite-keratophyre, com- 

 posed of albite laths and abundant magnetite. Albite pheno- 

 crysts occur in this, and may project, quite unaltered, right out 

 into the clear felspathic zone, or may even traverse two or three 

 zones, retaining the general trachytic direction of the nodule. 

 The above is the most complete type of polygonal nodule ; in 

 others less well developed, some of the zones may not be present. 

 In a few instances, the central magnetite-keratophyre, with its 

 more or less marked texture, is wrapped round by a zone of kera- 

 tophyre free from magnetite, in which the laths have a circum- 

 ferential arrangement. The boundaries of the several zones are 

 never sharp. Where spaces occur between the nodules, they are 

 filled with a mosaic of chiefly untwinned felspar. Except for 

 the abundance of magnetite, this rock is allied in structure with 

 the purely felspathic brecciated keratophyre last described. 



