24 



their varying mineralogical character, and, in fact, to be an effect of the same cause. As 

 the varying crystallization of the rocks seemed to reject the distinction of plutonic and vol- 

 canic so their structure nöt only approximates to that of both those divisions , but even 

 partakes , often in a striking manner , of that which characterizes some of the principal 

 members of what Mr. Lyell terms the metamorphic series. The alternation of beds or zones 

 of different composition, and the approach, where mica abounds, to the structure ofgneiss, 

 frequently assimilate them to rocks of that series; while the predominance of granitic types, 

 and the general character of the whole rocks, demonstrate their direct origin from igneous 

 fusion. The close approximation of this developement of igneous rocks at some of its points 

 to certain points in gneissose developments will be more particularly considered in the 

 sequel (1). 



Some geologists appear still to doubt whether granite ever has an original concretionary 

 structure. Thus Sir H. de ia Beche in his valuable Report on the Geology of Comicall, 

 Bevon and West Somerset (p. 450) in reference to the detached blocks, protruding rocks, 

 and hollows called rock basins, which abound in the granitic tracts of that district, says 

 that, after having given much attention to the subject, he is far from perceiving good evidence 

 in favour of the opinion that the globular blocks are owing to an original concretionary 

 arrangement of the granite. He adds, that after much careful observation he is inclined to 

 refer the rounded character of a large proportion of the blocks , either scattered over the 

 surface or still existing in the tors of the granitic districts , more to the decomposition of 

 surfaces produced by divisional planes than to any other cause. Many of the exposed granite 

 blocks on the Pinang mountains are rounded at the edges , and this is undoubtedly there 

 the result of mere weathering. That an originally angular block must disintegrate more 

 rapidly on the edges than elsewhere is evident, for there two faces both subjected to meteo- 

 ric action approximate and meet, so that the edge decomposes quite through, and being in 

 every fall of rain converted into the summit of a petty waterfall, the same mechanical pro- 

 cess which, on a grand scale , is wasting the cliffs of Niagara, wears down the decompo- 

 sing edge. But such an operation 'would not explain the occurrence of really globular blocks, 

 and as these are seen on Pulo Ubin in the very act of separating from the original compact 

 masses in which they had been formed , and exposing the concentric coats of which they 

 consist , the fact of granite assuming this structure under certain conditions is matter of 

 ocular demonstration. The fact, however, is not new, for although it is not noticed by 

 Lteli , Phihips , or any other of our recent English writers on the plutonic rocks with 

 whose works I am acquainted, and Sir H. be la. Beche is evidently not aware that a concen- 

 tric laminar structure had ever been actually observed in granite , it is distinctly menlioned 

 by one of Wernbr's pupils , the most able and learned of our few mineralogists , Professor 

 Jameson of Edinburgh , in his article on mineralogy and geology in the Edinburgh Ency- 

 clopcedia , and it seems to be also well known to many continental geologists. Professor 

 Jamesotï says. » Some granites are disposed in rounded balls or concretions , which are from 



(1) See Post p. 27. 



