2.50 . Mineralogkal Observations on Cornwall. [Oct. 



# ceive, lie among the transition rocks ; but as I did not see the 

 quarry, I can give no information respecting it. 



The clay slate and gray wacke extend as far south as the river 

 Hel, about four miles south of Falmouth. I crossed the mouth 

 of that river in the evening, landed at Portalla, and spent the 

 night in a fisherman's cottage. Next morning the clay slate was 

 no longer to be seen. The rocks at Portalla are porphyry. The 

 base is a compact felspar, approaching somewhat to hornstone 

 in its hardness and fracture. This- porphyry continued but a 

 short way as we walked south, and was succeeded by a beautiful 

 and very hard greenstone, interspersed here and there with beds 

 of greenstone porphyry. This greenstone appeared to me to 

 extend south about three miles ; but as I went along the sea 

 shore, and followed the windings of the coast, it was not easy 

 to form an accurate conception of the true breadth of the beds 

 over which I passed. 



The greenstone is followed by a very beautiful bed of diallage 

 rock. This rock has been frequently observed in other countries, 

 and is well known to be intimately connected with serpentine. 

 The Italians have given it the name of gablro ; but Von Buch, 

 who met with it in abundance in Norway, has distinguished it 

 by the name of diallage rock, a name which I have adopted, as 

 more suitable to our language than gabbro. It is composed of 

 compact felspar and diallage. In the peninsula of the Lizzard, 

 the diallage is usually brown, commonly with a shade of grey or 

 green. The felspar is sometimes dark coloured, sometimes white. 



The road by the sea-side in this part of the peninsula became 

 so difficult that I was obliged to leave it, and penetrate into the 

 interior of the country. The consequence was, that I missed 

 the termination of the diallage rock, and the commencement of 

 the serpentine, the bed which follows next in order ; for the 

 interior of this peninsula consists of a flat land, wholly covered 

 with heath, or grass, or corn ; so that the rocks are almost every 

 where hid from our view. The serpentine appeared to me to 

 commence between four and five miles north from the Lizzard 

 Point, and it continues to within about half a mile of the light- 

 house. The serpentine of the Lizzard is the common serpentine, 

 and is distinguished by the usual variety of colours of that beau- 

 tiful rock, but red is by far the most prevalent. On the sea 

 shore it is every where altered on the surface by the action of the 

 weather : it is covered by a thin white crust, and every where 

 split by innumerable rifts, which appeared to me to separate very 

 accurately all the different coloured pieces from each other. 



About half a mile from the Lizzard Point the serpentine ter- 

 minates, and is succeeded by a bed of clay slate, which con- 

 tinues to the seashore, and constitutes the most southern extre- 



