176 J. A. PHILLIPS ON THE SO-CALLED 



fathoms in width, and has a direction north of east and south of 

 west ; its dip and apparent cleavage very nearly coincide with those 

 of the enclosing slates. Although large quantities of stone have 

 at various times been removed from this locality, the quarry has not 

 been worked in such a way as to expose a complete transverse section 

 across the band. It is, however, evidently traversed for a con- 

 siderable distance by a nearly perpendicular fault ; and at a point 

 about two hundred fathoms south-west of the quarry it is inter- 

 sected by the Great Terrace Elvan. This elvan is remarkable from 

 the circumstance that many of its felspar crystals have become re- 

 placed by a mixture of schorl and cassiterite, which at different 

 periods has caused it to be worked as a source of tin-ore. 



A fresh specimen of this rock afforded by analysis the following 

 results (sp. gr. 3-03) : — 



I. II. 



Water i h yS rometric ' 20 ' 22 



Water \ combined -77 -67 



Silica 46-89 46-96 



Phosphoric anhydride *31 -32 



Alumina 20-46 20-53 



Ferric oxide 2-19 2-09 



„ persulphide traces traces 



Ferrous oxide 11-15 11*18 



Manganous oxide trace trace 



Lime 8-37 8-42 



Magnesia 5-83 5-91 



Potassa trace trace 



Soda • 3-87 3-80 



100-04 100-10 



Thin sections, examined microscopically, are not unlike those of 

 the rock from North Huel Rose, excepting that the hornblende is 

 disposed in more regular bands, and the magnetite is also more dis- 

 tinctly banded. The edges of the green hornblendic stripes are 

 fringed by feathery crystals of that mineral ; and the patches and 

 crystals of magnetite are often cracked, or have become replaced by 

 a siliceous mineral. 



Between the nearly parallel hornblendic bands before referred to, 

 the transparent colourless base is traversed by acicular hornblendic 

 crystals, the longer axes of which lie (mostly in one direction) in 

 planes coinciding with the cleavage. Viridite is always present in 

 small quantity ; and one of the sections prepared from this rock en- 

 closes small patches of a blue mineral, which is perhaps indicolite. 

 Among the hssile hornblendic rocks, several have been observed to 

 contain occasional small crystals of apparently a schorlaceous mineral. 

 Five sections of the Terrace blue elvan were cut and examined. 



It will be observed not only that the blue elvans, or greenstones, 

 occurring between St. Erth on the west and St. Stephens on the 

 east, although varying somewhat in the relative proportions of their 

 several constituents, resemble one another in their general chemical 



