36 Rev. J.J, Conyheare on the [July, 



Tariety or other of hornblende, of diallage, and occasionally per^ 

 haps of other minerals, which (as hypersthene) enter more spa- 

 ringly into the composition of rock masses. Such admixtures 

 can be properly studied only in those endless suites of specimens 

 which nature herself preserves, and presents in situ. The sub- 

 ject is an interesting one, and well deserves closer attention and 

 investigation than it has yet met with. The general character of 

 the gneiss in question, the brownish purple colour by which it is 

 almost every where distinguishable, its very limited extent, and 

 its gradual passage into the common killas, have been sufficiently 

 noticed by most recent observers. I do not recollect ever to 

 have seen any mineral substances imbedded in its mass. It is 

 of coi^rse traversed, like the adjacent rocks, by metallic and other 

 veins. This passes, as is well known, by a rapid transition into 

 the common killas or clay slate. Before entering on the consi- 

 deration of this, by far the predominant form of stratified rock 

 throughout the whole of the west, it may be well to notice two 

 of its subordinate beds, or, perhaps, varieties, which, though 

 much inferior in point of extent, yet present appearances much 

 more clearly indicative of their mineral composition, and capa- 

 ble, perhaps, of throwing some light on that of the common 

 killas with which they are so closely associated. 1. Common 

 chlorite slate. This needs no further description, and I am not 

 aware that it has ever been found to contain any other imbedded 

 mineral than the garnet, specks of iron, and perhaps of copper 

 pyrites. 2. A laminated rock, of a silvery grey colour, and 

 micaceous aspect, exhibiting throughout its mass small patches 

 of a darker tint, having the appearance of some imbedded mine- 

 ral obscurely crystallized, and much intermixed with the slate 

 containing it. These patches have been regarded as allied to 

 grepatite, to hornblende, and to some other mineral species, but 

 ploser exainination shows them (unless I be mistaken) to consist 

 of a dark-grey chlorite minutely and confusedly crystallized. 

 This variety of killas contains in the neighbourhood of Camel- 

 ford, where it may perhaps be most advantageously studied, 

 small contemporaneous veins of crystalline felspar ; and in one 

 or two instances alternates with thin beds of compact felspar 

 tinged by the admixture probably of chlorite ; a circumstance 

 observable also in the killas of Wheal Maudlin ; and in that 

 which succeeds the granite near Ivy Bridge; though, in these 

 atter cases, it is possible that the penetrating matter may be 

 hornblende.* 



But these varieties are, as I have stated, but of partial occur- 

 rence and limited extent. The stratified rock of the mining 

 district is almost univerdally the common killas. This rock, 

 after much question (which your readers would scarcely wish to 



• I may here observe that the neighbourhood of Ivy Bridge offered by far the most 

 beautiful and characteristic specimens of compact felspar uiioltered a|)parently by any 

 mixture, which I ever met with in the west, ' ' 



