4 PE0E. A. H. GREEN ON SOME RECENT [Feb. 1895, 



the character of the rock. Another hand, somewhat coarser in 

 grain, but otherwise similar to the last, I was able to trace from the 

 south-western side of Tinker's Hill, across the valley, on to Broad 

 Down. Many other rocks of a similar character, some banded and 

 showing ill-defined fluxion-structure, occur along the same belt. 



East of these a rock with fragmental texture occurs on the 

 northern side of Broad Down, and a rock very similar in character 

 crops out on the same strike on the southern side of that hill. I 

 think that no one possessing a reasonable amount of caution would 

 speak with certainty about the rocks of this belt ; but the probability 

 is that we have here a series consisting of alternations of rhyolitie 

 lavas and tuffs. The exact localities of the specimens analysed by 

 Mr. Timins cannot be fixed, but it is not unlikely that nos. xli. to 1. 

 belong to this belt. In it lies the rock in which Mr. Rutley believes 

 that he detected perlitic structure. 



A very singular rock belonging to the same belt was exposed in 

 the cutting for the diverted road. The matrix may contain some 

 isotropic matter, but it is thickly crowded with very minute doubly- 

 refracting grains and hairs. Its peculiarity consists in the presence 

 of a large number of ring-shaped patches, edged with a mineral 

 that polarizes in brilliant colours, the centre of each ring being like 

 the matrix, but rather darker in colour. This rock was analysed 

 for me by Mr. B. Hornby (Queen's College, Oxford). As it was 

 thickly veined with calcite the powder was first treated with cold 

 dilute hydrochloric acid for 24 hours. The insoluble residue had 

 the following composition : — 



Silica 78-41 



Iron and alumina 16 - 38 



Magnesia 088 



Lime 055 



Alkalies, by difference 3 - 78 



My friend Mr. Butley has spent some time over this rock, but 

 without coming to any definite conclusion as to the meaning of its 

 peculiarity. 



Next follows a belt of dolerite, which resembles the rock of 

 Clutter's Cave. It was well shown in the cutting for the diverted 

 road at the south-western end of Tinker's Hill, in a quarry on the 

 opposite side of the valley, and in the cutting for the catchwater. 

 It is traceable up to the summit of Broad Down, and indications of 

 a similar bed on the same strike were seen on the southern slope of 

 that hill. The bed seemed here to be thinner than on Tinker's 

 Hill, and I could find no trace of it on the opposite side of 

 the valley on Hangman's Hill. At one spot on Tinker's Hill 

 Mr. H. D. Acland called my attention to parts of this bed which 

 are vesicular and amygdaloidal. 



East of the band of dolerite last described the rocks look, to the 

 unaided eye, very much alike — hard, splintery, and felsitic in aspect. 

 Those which I have examined under the microscope are some of 

 them fragmental, while others have the rhyolitie character of the 

 rocks to the west. Probably we have here a second belt of rhyolitie 

 lavas and tuffs. 



