196 GENERAL REMARKS [Ch. X. 



quently, several are found united, not only in the same layer, 

 but even in an individual concretion ; passing into each other, 

 at some points, by the most imperceptible gradations, whilst 

 at others they appear disunited by an open seam on the line 

 of transition. Under these circumstances, they are disposed 

 in the forms of courses, veins, or stripes, either parallel with 

 the layers, or crossing them at various angles; either very 

 large, so as traverse several layers, or small, and confined to 

 one or more blocks or concretions of which the layer or bed 

 is composed ; or they constitute distinct nodules or spheroids 

 of various sizes, which sometimes consist of concentric laminae ; 

 but, more commonly, they occur in the state of spots and 

 patches, very irregular both in size and shape. These differ- 

 ent forms of granite are not only connected with the pre- 

 vailing variety by mineral transitions, even within distinct 

 concretions ; but it sometimes happens, that the same por- 

 phyritic crystals belonging to the main mass are partly im- 

 bedded in the patches, veins, and courses of the associated 

 granitic species. 



Rocks of precisely the same composition as those just 

 enumerated commonly occur on a smaller scale, in the crys- 

 talline slates adjacent to the large mass or group of granitic 

 rocks ; and under precisely the same circumstances of dis- 

 position in relation to the slates, as has just been stated to 

 subsist between the subordinate and predominating varieties 

 of the main mass of granite. For example, they occur in 

 regular layers or beds, alternating with the primary strata, in 

 Cornwall, Ireland, Germany, and elsewhere : and these 

 granitic rocks are always of the same nature as those which 

 form a component part of the adjoining mass of granite; and 

 have been observed in Cornwall to pass frequently into the 

 slate by mineral gradations ; and not only so, but in addition 

 to this, they sometimes are continuations of the same in- 

 dividual beds contained in the main granite. Again, they 

 are not only disposed in the slate in regular beds, but also 

 in courses, veins, patches, and nodules of divers forms and 



