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plicity of the language in which they are expressed) in the Chapter of his Elements in which 

 he treats of the plutonic rocks, and in which he refers them to a deep subterranean source, 

 quotes the following passages from Dr. MagGuilogh's System of Geology. 



«The ordinary granite of Aberdeenshire is the usual ternary compound of quartz, felspar, 

 and mica ; but sometimes hornblende is substituted for the mica. But in many places a 

 variety occurs which is composed simply of felspar and hornblende; and in examining more 

 minutely this duplicate compound, it is observed in some places to assume a fine grain , 

 and at length to become undistinguishable from the greenstones of the trap family. It also 

 passes in the same uninterrupted raanner into a basalt , and at length iato a soft claystone , 

 with a schistose tendency on exposure, in no respect differing from those of the trap islands 

 of the western coast." The same author mentions , »that in Shetland , a granite composed of 

 hornblende , mica ; felspar, and quarlz , graduates in an equally perfect manner into basalt." 

 Mr. Ltell continues: » In Hungary there are varieties oftrachyte, which, geologicaliy speak- 

 ing, are of modern origin , in which crystals not only of mica but of quartz are com- 

 mon , together with felspar and hornblende. It is easy to conceive how such volcanic mas- 

 ses may , at a certain depth from the surface , pass downwards into granite" (1). 



This is very true, but although the facts previously cited from Dr. MacGullocii prove the 

 easy gradation between basaltic and granitic rocks , they seem as little reconcileable as the 

 rocks of Pulo Ubin are with the theory of a necessarily deep tartarean origin of the latter, 

 and less subterranean origin of the former. In Singapore there are blocks of greenstone as 

 highly crystallized as any granite but in which large isolated portions become compact and 

 approach to basalt. In these cases it is impossible to refer the change from a compact to a 

 crystalline structure merely to the mechanical pressure of the superincumbant crust, for such 

 a force must have acted uriiformily throughout the whole mass before solidification. The 

 cause of the variations in the igneous rocks (excluding those which cool in or near the 

 atmosphere), must be chemical or electiical more than mechanical. It is probable that Mr. 

 Darwiiv, in drawing attention to the established influence of disturbance in causing certain of 

 the ingredients of a molten mass of different elements to crystallize and separate from the 

 mass , and thereby attain the power of ascending or descending through it to a different 

 level according to the specific gravity of the crystals , has discovered a very potent mecha- 

 nical agent which is subservient to the chemical causes of the gradations in the plutonic and 

 volcanic rocks. While believing, however, that the influence of continued uniform pressure 

 (and this implies the absence of disturbance) will be chiefly exhibited in communicating a 

 homogeneous character to the fluid rock solidified under it , I would not be understood to 

 limit the effect of disturbance to the mere separation of ingredients by their different rela- 

 tive gravities. It ratlier appcars to me that a very important operation of mechanical agi- 

 tation has been overlooked by Mr. Darwin. 



This leads me to notice the next remarkable feature of the Pulo Ubin rocks , their cuboi- 

 dal , globular _, laminar and zoned struture , which I conceive to be intimately connected with 



(1) Ltbll's Elements of Geology (2d ed.) vol. 2 p. 334. 



