40 DESCRIPTION OF THE PRIMARY [Ch. IV. 



hard to very hard ; and breaks, though not easily, into thick 

 plates full of joints; its surface is often spangled with a 

 micaceous mineral, the parallel arrangement of which imparts 

 to the mass a laminated structure : on decomposition, it be- 

 comes much more schistose, and of a dirty yellowish white 

 colour. 



Proteolite, as its name is intended to indicate, assumes a 

 great variety of forms. In colour it does not much differ 

 from the rock just described, but its tints are generally much 

 lighter ; indeed, its original colour is not often met with, for, 

 owing to the tendency of this rock to undergo a partial 

 decomposition, becoming earthy and argillaceous, it is ge- 

 nerally found of a yellowish or very pale brown colour. It is 

 much softer than cornubianite, and its hardest kind, instead 

 of possessing the fine compact texture of that rock, is always 

 more or less arenaceous. Its species are much more schistose ; 

 its composition is, for the most part, pretty uniform ; some- 

 times, however, a shining mineral is present, in minute spots 

 and scales, and in one instance the whole basis is shining and 

 glossy, approaching very near to mica-slate. 



Both of these slates appear to be principally composed of 

 compact felspar, which is probably a compound, as already 

 stated, of felspar and quartz in various proportions : in 

 cornubianite, this substance has an uniform massive texture; 

 in proteolite it is finely granular. And, as the felspar or 

 quartz predominates in the compact felspar basis, so these 

 rocks exhibit different degrees of hardness, and other pro- 

 perties characteristic of the prevailing constituent By these 

 fluctuations in the composition of the basis, we can easily 

 comprehend how these rocks sometimes contain beds of pure 

 compact felspar, at others, beds which are entirely quartzose. 

 The colouring or accessary mineral of these slates is not 

 always obvious, but it appears to be either mica or shorl, or 

 some intermediate substance. 



It may be enquired, how can this be ascertained ? The 

 nature of the colouring mineral can only be proved in this 



