1854.] HORNER IGNEOUS ROCKS IN CAWSAND BAY. 361 



examined at low water, and looking over its broad surface, the por- 

 phyry is seen to be traversed in all directions by joints, which divide 

 the rock into wedge-shaped and polygonal masses of various dimen- 

 sions. A prevailing series of the joints have a W.N.W. direction, 

 corresponding with that of the slates on the west, giving the por- 

 phyry the appearance of being composed of highly inclined beds. 

 On measuring it by walking in a line perpendicular to its western 

 side, I counted 1100 paces until I again came to slate eastward, so 

 that this mass of igneous rock cannot be less than half a mile wide, 

 and I found it identical in nature on both sides*. In some places it 

 presents a perfectly flat surface of great extent, which in one part I 

 estimated to include not less than 2000 square yards. 



The base of this porphyry is a purplish-red felspar, interspersed 

 vnth minute dark green crystals of hornblende and some specks of 

 mica. Steatite appears to form a constituent of the base, and it is 

 disseminated through it in white and coloured spots, and seems also 

 to coat the joints. Under the blowpipe the reddish base changes to 

 greenish-white, similar to the spots, and is fused vdth great difficulty. 

 One of the most striking peculiarities of this porphyry, and which at 

 first sight gives it the appearance of a red sandstone, is the predo- 

 minance of white and bluish-green stripes, patches, and spots in every 

 partf ; but all these have a much greener aspect in the rock than 

 they preserve in hand specimens, after a little while ; which may 

 arise from humidity when in the rock. The surfaces of the joints 

 are very generally greenish-white ; sometimes it is a mere film, at 

 other times the change of colour has a depth of three or four inches. 

 So much do the stripes and patches of the lighter colour give the 

 rock the appearance of a red sandstone, that it is not until it is 

 struck with the hammer that we discover that it is not a sedimentary 

 deposit. 



It is well known that the rocks of Cornwall are traversed by nu- 

 merous porphyritic dykes, provincially called ' Elvans.' They are 

 often vei'y narrow, but have been seen as much as 400 feet wide ; 

 many of them may be followed for several miles, and one extends 

 from the neighbourhood of Marazion to Camborne, a distance of 

 twelve miles;];. They consist of a felspatho-quartzose base, including 

 crystals of quartz and felspar, and occasionally specks of mica and 

 hornblende or schorl. As the constituents are mixed, as in other 

 igneous rocks, in a great variety of proportions, the rocks have many 

 diflFerent aspects, but still they have a common mineral character. 



I have examined the extensive series of specimens of Elvans in the 

 Museum of Practical Geology, but have not found any exactly cor- 

 responding to the porphyry of Cawsand Bay ; those most resembling 

 it are the felspar porphyries of Creegbroaze Quarry, Chacewater, of 

 Pentewan St. Austell, and of Tremore near Bodmin. 



The slate rocks on the western side of the porphyry are in many 

 places very much disturbed and contorted; sometimes they are vertical 



* Specimen No. 1, from the eastern side. That from the western side has been 

 unfortunately lost. [The specimens are in the Museum of the Society.] 



t Specimen No. 2, western side. J Report on Cornwall and Devon, p. 174. 



2 c 2 



