486 J. A. PHILLIPS ON THE SO-CALLED 



affording an example of rocks chemically alike, differing materially 

 in their mineralogical constitution. 



A little north of the main road at Two Bridges is a quarry worked 

 for road-material, upon a rock which is a scarcely altered dolerite. 

 The principal indication of change is the replacement of a large 

 proportion of the ilmenite by the usual greyish-white material and 

 the presence of patches of viridite resulting from the decomposition 

 of augite. 



Eather more than a hundred yards east of the church of South 

 Petherwin, and immediately north of the highroad, a large quarry 

 has been opened upon a band of fine-grained greyish " blue elvan," 

 which occasionally encloses spots of iron pyrites. An examination 

 of thin sections shows that it consists, to a large extent, of unaltered 

 plagioclase, with which is associated equally fresh augite. The 

 ilmenite has undergone the usual change ; but in other respects this 

 rock, which is a typical dolerite containing no visible apatite, is 

 unaltered. 



Although " dunstone " may probably occur in this district, that rock 

 was nowhere observed. 



South-eastern District. " Blue elvans." — Writing about the 

 year 1822, the Bev. John Eogers says, " The hornblende formation 

 of St.-Clere (near Liskeard) is principally confined to that elevated 

 land called St.-Clere Down. It extends about a mile from east to 

 west and about half a mile from north to south, and appears to run 

 east and west, and to dip towards the south or south-west. 

 It may be observed that hornstone and hornblende- 

 slate, passing into clay-slate, are found throughout the whole line of 

 junction [of granite and clay-slate], except towards the north-east, 

 where I found no hornstone. In that direction hornblende-slate, a 

 kind of greenstone-slate, and clay-slate, abounding with particles of 

 mica, form the connecting chain of minerals from hornblende to 

 granite. In this quarter, likewise, in a field adjoining the vicarage 

 garden, towards the west, asbestus has been found in veins in green- 

 stone-slate ; but the quarry in which it was found has been filled up 

 for some years. 



" The mineral which I have called hornblende-slate graduates so 

 imperceptibly into clay-slate, that! have hesitated which name to 

 give to some of the specimens ; it appears to be the slaty felspar 

 rock of Jameson, and has been sometimes denominated purple 

 killas"*. 



A specimen of the crystalline coarse-grained rock from St.-Cleer 

 Down, analyzed, in duplicate, in the author's laboratory, by Mr. J. 

 H. Brown, afforded the following results, showing that it has the 

 usual chemical composition of this variety of greenstone (sp. gr. 



* " Observations on the Hornblende Formation in the parish of St. Clere," 

 by the Rev. John Rogers (Trans. Royal Gj-eol. Soc. of Cornwall, vol. ii. 

 p. 218). 



