324 J, ARTHUR PHILLIPS ON THE ROCKS OF 



with dark spots, which a higher power shows to consist of aggrega- 

 tions, about y^j-y of an inch in diameter, of hexagonal plates of 

 micaceous iron. In addition to these, transparent belonites tra- 

 verse the rock in all directions, hut principally in planes parallel 

 with the cleavage. These, as well as the crystals of oxide of 

 iron, are evidently not merely imbedded,, but have been formed in 

 situ*. 



VIII. Killas from Botallaclc, Penzance ; surface near lode. — This 

 rock, which occurs between the granite and the band of " greenstone" 

 forming some portions of the cliff in that part of Cornwall, is of a 

 dark greenish-grey colour, and is very hard. Its cleavage is to a 

 great extent obliterated ; but when broken it nevertheless divides 

 into roughly tabular masses. Under the microscope, sections of this 

 rock are seen to consist of a transparent base, through which horn- 

 blende and a fl. occulent greenish mineral are distributed, and in which 

 are imbedded a few minute belonites. Patches of magnetite and 

 a few small crystals of iron pyrites are disseminated throughout. 



IX. Bach from Botallaclc, One-hundred-and-ihirty-fatJiom level. — - 

 This specimen, sent under the name of " killas," has lost all traces 

 of cleavage, and breaks with the polished surfaces so generally ob- 

 served in serpentinous rocks. It has a dark-green colour, and 

 slightly attracts the magnetic needle. It will be remarked that both 

 specimens of the so-called killas from this mine contain a large 

 amount of magnesia, which is almost entirely wanting in the clay- 

 slates from the neighbourhood of St. Austell. Under the microscope, 

 this rock is seen to consist of a transparent base, permeated by 

 minute crystals, apparently of hornblende, whose larger axes cross 

 each other in all directions. In addition to these disseminated 

 hornblendic crystals, there are patches made up of bundles of similar 

 crystals, of which the longer axes are arranged in the same direction. 

 Magnetite is disseminated in granular patches throughout the rock. 

 Sections of several of the neighbouring rocks show that hornblende 

 is sometimes more or less replaced by chlorite, and that the amount 

 of disseminated magnetite is very variable. 



X. Killas from Huel Seton, Camborne, One-hundred-and-sixty- 

 fatliom level. — This is a very hard grey clay-slate, of which the 

 cleavage has, in the majority of cases, been to some extent ob- 

 literated by metamorphism. No crystalline structure can be de- 

 tected by the eye ; and the rock, in addition to being traversed by 

 strings of white quartz, encloses minute spots and crystals of iron 

 pyrites. Sections are seen under the microscope to consist of bands 

 of transparent granular quartz, alternating with layers of similar 

 quartz, through which minute hornblendic crystals are dissemi- 

 nated. 



It is worthy of notice that while the slates of Botallack are highly 

 magnesian, the sea-water which percolates through them into the 



* Almost all the specimens of Cornish killas which have been examined have 

 been found to contain either magnetite or more or less numerous flakes of specu- 

 lar iron ore. 



