﻿Microscopic Constitution of certain Cornish Rocks, 97 



which are brilliantly coloured by polarized light. Like the brown 

 crystals, occurring in groups, these are not mechanically em- 

 bedded, but have been formed in situ since the deposition of the 

 clayey matter of which the rock is mainly composed. 



Diorite, "Sanctuaries," St, Mewan. Sp. gr. = 2'97. — On the 

 map of Cornwall, prepared by the Officers of the Government 

 Geological Survey, a dyke of " greenstone " is laid down as cour- 

 sing in a north-easterly direction from the farm of Quoit to a 

 point about a quarter of a mile north of the turnpike-road from 

 Truro to St. Austell, where it expands into a large mass at the 

 summit of the hill ; thence it extends, in diminished proportions, 

 in a south-easterly direction to the sea at Duporth. This rock, 

 which is of a dark green colour verging on black, is exceedingly 

 hard and tough, and is extensively worked, at the point of its 

 greatest expansion, as a material for making roads. The quarry 

 from which this stone is obtained adjoins the " sanctuary fields " 

 belonging to the glebe of the parish of St. Mewan, and has long 

 afforded a road-material of such remarkable excellence that it is 

 carted to considerable distances to be employed for that purpose. 

 In the quarry above referred to, this rock is distinctly crystal- 

 line, frequently encloses patches of iron pyrites, and attracts the 

 magnetic needle. In some smaller openings which have been 

 made upon it on its south-eastern extension, its crystalline cha- 

 racter has to a considerable extent disappeared, and pyrites has 

 been replaced by oxide of iron. Fragments of rock free from 

 pyrites were chosen for analysis ; and three separate analyses were 

 made, with the following results* : — 



I. II. III. 



Water t *83 -81 '76 



Silica 47-66 47*33 47-70 



Titanic acid .... trace trace trace 



Phosphoric acid . . *16 '18 trace 



Alumina 17'50 17*15 16-83 



Ferric oxide .... 12*52 13-18 13-42 

 Ferrous oxide . . . 942 9*42 9*07 



Oxide of manganese ... . . trace 



Sulphur trace trace trace 



Lime 4'20 4-03 4*10 



Magnesia .... trace trace trace 



Potassa 2-43 2'33 2*15 



Soda 5-19 5-27 5-88 



99-91 99-70 99-91 



* Analyses I. and II. were made on the same fragment of rock ; No. III. 

 on a different one. 



t Of which "33 was lost in the water-bath. 

 Phil. Mag. S. 4. Vol. 41. No. 271. Feb. 1871. H 



