104 THE STRUCTURE OF THE [Ch. VI. 



latter is only to be distinguished from that of granite by its 

 foliated modifications predominating over the massive, whereas 

 it is the reverse in granite. 



The various kinds of structure so frequently pass into each 

 other in the same mass, that it is oftentimes very difficult to 

 observe where the one begins and the other ends. When the 

 rock has a compact texture, but more particularly if it be 

 crystalline, it most commonly exhibits a massive structure; 

 and as it gradually becomes more and more fine-grained and 

 homogeneous, it generally acquires a proportional degree of 

 fissility, acquiring at last a perfect slaty structure : on this 

 account it is often impossible to draw a line of demarcation 

 between the true slates and the massive, crystalline, and gra- 

 nitic rocks, with which they are associated ; since they gra- 

 dually pass into each other in their mechanical arrangement, 

 as well as in their mineral composition. This is not only the 

 case in Cornwall, but in all primary districts. It would lead 

 us into too many details, and into many repetitions, if we 

 were to enter fully on this subject ; let it therefore suffice to 

 quote one example from Dr. Macculloch, on Scotland. " The 

 primitive greenstone and greenstone- slate consist of mixtures 

 of felspar and hornblende ; in the latter case disposed in a 

 laminar form, and in the former, without that regular struc- 

 ture." " These greenstones constitute a class of rocks entirely 

 different from those that are intruded amongst the regular 

 system of secondary strata; and have evidently been formed 

 together with the gneiss and mica-slate which they accompany ; 

 and are, like them, referable to the same epoch, never intrud- 

 ing to their disturbance, but occupying situations and main- 

 taining characters in every respect conformable to them."* 



Thus we find that the internal structure of the primary 

 rocks, both granitic and schistose, is of the same nature ; the 

 only difference that can be observed is, that the massive 

 Structure prevails amongst the former, and the latter rocks 



* Gaol. Trans., vol. ii. p. 393. 



