the Waters of a Salt Spring in Huel Seton Mine. 29 



I. II. 



Water* 1-53 156 



Silica ...... 70-76 70-85 



Alumina 1678 16*86 



Ferrous' oxide . . . 2*44 250 



Manganous oxide . . 1*24 1'18 



Sulphur trace trace 



Lime 1-23 1-21 



Magnesia 1*55 1*52 



Potassa ..... 3-08 3-20 



Soda -85 -92 



99-46 99-80 



Under the microscope this rock is seen to consist of a felsitic 

 base containing crystals of orthoclase. The base is composed 

 of highly altered felspar, pseudomorphs, apparently after horn- 

 blende, and numerous sphserulites exhibiting a radial structure. 

 These sphserulitic bodies are not terminated by sharp outlines, 

 but become gradually blended with the surrounding base ; their 

 radial structure, however, is perfectly defined, particularly when 

 examined by polarized light. There is a green flocculent sub- 

 stance disseminated throughout the base; and many small patches 

 of quartz are observed, which, by polarized light, appear to break 

 up into granules. There are also prisms of a green colour, 

 which are apparently altered hornblende; in a few cases one 

 portion of a crystal is seen to be composed of some hornblendic 

 mineral, while the remainder has been replaced by felspar. In 

 some of the sections examined minute spots and microscopic 

 crystals of iron pyrites were observed. 



This rock would be generally described as a porphyry; but 

 Mr. S. Allport, who at my request had the kindness to cut and 

 examine a section of it, considers its composition would be better 

 indicated by the name Porphyritic Felsite. It has, however, 

 undergone a considerable amount of alteration; and although 

 the larger crystals of orthoclase are for the most part unchanged, 

 the smaller ones are almost without exception pseudomorphs. 



Granite from Cam Brea, sp. gr. = 2'64. — The Cam Brea Hill, 

 about a mile south-east of Huel Seton, is composed of a rather 

 coarse-grained granite, in which the proportion of quartz is gene- 

 rally large ; the felspar sometimes occurs in crystals of consi- 

 derable size. It contains two varieties of mica, the one black 

 (lepidomelane ?), and the other white or slightly tinged with 

 pink ; the latter appears to be a variety of lepidolite. 



The chemical composition of this granite was found to be as 

 follows : — 



* Of which '19 was lost in the water-bath. 



