24-2 AN ENQUIRY INTO THE [Ch. XII. 



tracing any particular bed, it will very frequently be found, 

 within no considerable distance, to have passed into a different 

 kind of rock. In fact, they are constantly changing their 

 mineral character; and though on the large scale different 

 kinds of gneiss, mica-slate, hornblende-schist, and clay-slate 

 alternate with each other, yet the continuation of either, 

 laterally, often assumes the appearance of all these rocks in 

 detail. In this respect there certainly seems to be no great 

 similarity between the crystalline and fossiliferous strata. 

 But how does the case stand when we compare the primary 

 slates with the granitic rocks ? 



If we traverse any considerable mass of granite which 

 offers favourable opportunities for its examination, we find, 

 as in the case of the primary slates, a certain succession of 

 beds or layers, differing from each other in composition ; and 

 not unfrequently this series is repeated over again and again ; 

 that is, presenting precisely the same phenomenon which 

 among the secondary rocks would be termed an alternation 

 of strata. Thus, in crossing the central mass of granite due 

 north from the neighbourhood of St. Austle, in Cornwall, a 

 granitic rock first presents itself, which is composed of felspar, 

 quartz, and shorl (a granitic shorl-rock) ; and this is varied 

 by parallel layers, sometimes of considerable thickness, of 

 granular or compact shorl-rock, consisting only of quartz and 

 shorl ; to these succeeds shorlaceous granite, a rock which 

 contains shorl in addition to the common ingredients, felspar, 

 quartz, and mica, and this rock again gives place to various 

 kinds of true granite : still proceeding, we find the fine- 

 grained granite pass into china-stone (in which the mica is 

 replaced by talc), a rock fairly entitled to the name of proto- 

 gine ; and this is frequently traversed longitudinally by beds 

 of shorl-rock, shorlaceous and common granite. In this man- 

 ner the series is repeated, with greater or less regularity, in 

 several portions of this great mass which have been exposed 

 in the search after china-clay ; and it is, therefore, reasonable 

 to suppose, that the portions that are concealed from our 



