116 THE STRUCTURE OF THE [Ch. VI. 



substance, the rock passes insensibly, without any change in 

 its structure, or in the degree of inclination of its strata, into a 

 steatitic schist. And he remarks, " that this schist also contains 

 numerous nodules of quartz, which, being penetrated by the 

 matter of the talc, cannot be considered as rolled pebbles, but 

 as nodules cotemporaneous in their formation with the rock 

 itself." * Lastly, Mr. Weaver states, that, near Cronebane in 

 Ireland, the clay-slate occurs in almost every stage of union 

 with quartz, which varies in size from large compressed 

 lenticular nodules to the smallest grains, terminating in a 

 most perfect intermixture of the quartz with the substance of 

 the slate. These nodules are arranged parallel with the 

 direction of the strata ; and they often terminate in thin edges, 

 diffused between the laminae of the slate, f 



The primary rocks which are composed of angular con- 

 cretions, are of more frequent occurrence in Cornwall than 

 those of a conglomerated structure. The quartzose varieties 

 of the granitic rocks, in which quartz predominates, often 

 exhibit an angular appearance ; and this is much more dis- 

 tinctly developed in the same species of the primary schists. 

 The layer of felspathic rock, which is worked for tin ore near 

 the Indian Queens, in the central district, so abounds in 

 small angular portions of different colours and texture, that 

 it resembles a coarse lime-ash floor : this rock appears to be a 

 regular elvan-course, equivalent to the beds of porphyry 

 which are associated with the Cornish slates. At Huel Virgin 

 mine, near Marazion, one of the beds of slate is intersected 

 by small irregular veins of flesh-coloured felspar and quartz ; 

 and, in some parts, the former so predominate in the mass, 

 that the slate is divided into small angular portions. This rock 

 assumes every variety of form, according to the proportion of 

 its constituent parts, exhibiting a most perfect likeness of a 

 brecciated marble. This bed, according to Mr. Kenwood, is 

 considered as one of the lodes (as in the case of Relistian 



* Geological Essay, p. 83. f Geol. Trans, vol. v. p. 171. 



