Ch. XIV.] OF THE STRATIFIED ROCKS. 321 



morphosis, also renders it probable that the additional 

 element, alkali, could not have proceeded from the same 

 source. For, if clay-slate has been converted into hornblende 

 and other crystalline schists, and the latter contain an alkali 

 which is wanting in the clay-slate, and if the last-mentioned 

 rock also alternates with mica-slate and even with gneiss, 

 it is somewhat difficult to conceive how the alkali could have 

 been transmitted through these interposed beds of clay-slate. 

 The value of this argument rests on the composition of clay- 

 slate, and we have made use of it on the generally received 

 opinion of the nature of clay-slate : but we do not attach any 

 importance to this example, because we do not subscribe to 

 the general notion concerning this slate; indeed, the term 

 clay-slate has been so commonly and vaguely applied to most 

 schistose rocks of an uniform and fine texture, that it is, in 

 fact, perfectly useless, as it conveys no definite meaning. On 

 the contrary, we think that all true primary clay-slates, that is, 

 such as are associated and alternate with gneiss and analogous 

 rocks, do really contain an alkali ; that their basis is a com- 

 pact felspar, varying in composition, sometimes silica and 

 sometimes felspar prevailing : it is probable that alkali may not 

 be found in every specimen of this rock ; for, sometimes, the 

 clay-slate has assumed the character of a sedimentary rock by 

 a partial decomposition which has altered its appearance, and 

 may have removed the alkali without destroying the tenacity 

 and structure of the slate : this state of things often happens 

 in Cornwall, and is probably of universal occurrence. 



But in offering to withdraw the objection urged against the 

 transfusion of alkali through the primary slates, in the case 

 of clay-slate, it is not intended to drop it altogether, but to 

 claim its validity in the instance of quartz -rock, which, even in 

 a state of considerable purity, and often in beds of no incon- 

 siderable thickness, alternates not only with gneiss, but also 

 with every member of the primary rocks. Let the appeal be 

 made to the chemist, and by his answer let the question stand 

 or fall. Can potassa or soda be transmitted through a mass of 



