496 J. A. PHILLIPS ON THE SO-CALLED " GKEENSTONEs" 



and the observations made during the progress of the present in- 

 vestigation are in general confirmatory of the views entertained by 

 that accurate observer. 



Considering the early date at which the Geological Map of the 

 county of Cornwall was prepared, and at the same time making due 

 allowance for the fact of its being the almost unassisted work of a 

 single individual, the rocks marked " greenstones " have been laid 

 down with remarkable accuracy, although it is true that no distinc- 

 tion is made between lavas, ash-beds, and crystalline traps. The 

 " greenstones " have, however, been made to comprehend not only 

 some altered rocks of sedimentary origin, but also certain hornblendic 

 slates of which it would be difficult to ascertain the original con- 

 dition. It is almost needless to add that railway-cuttings and 

 other workings made since the date of the survey have exposed 

 masses of rock not then known to exist. 



DESCRIPTION OF PLATES XX.-XXII. 



Fig. 1. Section of fissile hornblendic rock from Trenewth, near Camelford, 

 magnified 22 diameters (see p. 481). 



2. Cavity in lava from Pentire Point, filled with quartz and vermicular 



chlorite, magnified 30 diameters (see p. 483). 



3. Lava from Bokelly, St. Kew, enclosing crystals of felspar, magnified 22 



• diameters (see p. 484). 



4. Section of the same rock, showing crystals of altered felspar which have 



become sphserulitic, magnified 14 times (see p. 484). 



5. Section of a dolerite from St. Minver, near Wadebridge, showing the 



gradual conversion of augite into uralite, magnified 14 times (see 

 p. 478). This rock is composed of a triclinic felspar, of which a 

 large proportion has become much altered, with augite, hornblende, 

 ilmenite, and occasionally apatite. In the lower portion of the field 

 will be seen a large crystal of partially altered ilmenite, while the 

 augite will generally be observed to have become converted into 

 hornblende, chiefly along some of its edges. In another case this 

 change has begun within the crystal itself. 



6. Section of rock from Treglynn Common, west of Scaddick Tor, showing 



bacillar hornblende, viridite, and altered ilmenite, magnified 14 dia- 

 meters (see p. 485). 



Discussion. 



The President expressed his gratification at so valuable a paper 

 being brought before the Society, its importance being greatly en- 

 hanced by the working out of the subject both chemically and 

 microscopically. Its peculiar value lay in the demonstration that 

 the ancient lavas came so near to the modern ones. 



Mr. Waringtok W. Smyth said that Mr. Phillips had changed 

 our previous ideas and implanted new ones. One of these was the 

 similarity of the constitution of rocks, some of igneous, some of 

 metamorphic origin, which he thought justified the course taken 

 long ago by Sir Henry De la Beche, who called all these rocks 

 "greenstones." Mr. Smyth also directed attention to the large 

 number of originally pyroxenic rocks in this district, which militated 



