BUNSEN — GRANITE, 11 



animals being decomposed by atmospberic agencies far more rapidly 

 tban tbat derived from vegetables. Tbe quantity of azote in such 

 rocks as clay-slate and cbiastolite-slate is very striking, and in some 

 cases amounts to as much as 0*29 ; yet such metamorpbic crystal- 

 line rocks as talc-, chlorite-, or mica-schist and gneiss contain mere 

 traces, and thus it should appear that the organic matters have been 

 partially or completely destroyed where metamorpbic action has 

 been very energetic. 



On the whole, the author has done good service for geology in de- 

 termining the quantity of azote in a large number of minerals and 

 rocks, as well as of other substances interesting in connexion with 

 them ; and although the amount may appear small, yet it is quite 

 clear that its presence or absence must henceforth be borne in mind 

 in speculating on the origiu of rocks or on the changes that have 

 since occurred in their chemical and physical constitution. 



[H. C. S.] 



On the FoEMATioif of Gea^te. By E. Buis^seit, of Heidelberg, 

 Por.M.G.S., &c., &c. 



[Ueber die Bildmig des Grranites. Zeitschrift der deutschen geologischen Ge- 

 sellschaft, 1861, vol. xiii. pp. 61-64; and Neues Jahrbuch fiir Mineralogie &c., 

 1861, pp. 856-^58.] 



A STEAi^GE error has for a considerable time played a great part in 

 the geological hypotheses of the formation of granite, the correction 

 of which will be to a certain extent a satisfaction to those geologists 

 who think that the inferences derived from careful and well-grounded 

 observations are threatened by the conclusions of experimental che- 

 mists. Quartz solidifies from fusion at a higher temperature than 

 orthoclase, and orthoclase at a higher than mica. If then, assert 

 the opponents of tbe plutonic origin of that rock, granite originated 

 from a mixture of those three minerals in a state of igneous fusion, 

 on cooling the quartz would soHdify first, then the felspar, and last 

 of all the mica. However, since the petrographical structure of 

 granite usually indicates a difi'erent order of solidification, they 

 maintain, further, that it was not of igneous origin. It is indeed 

 difficult to understand how for so many years geologists should have 

 considered such an erroneous conclusion of any value, and it is still 

 more difficult to comprehend how even now-a-days it is still repro- 

 duced in support of geological hj^potheses. No one appears to have 

 taken into consideration*, that the temperature at which a substance 

 sohdifies when fused alone is never that at which it solidifies when 

 deposited from solution in another substance. The temperature at 



* This remark is not strictly applicable in the case of Enghsh geologists, since 

 Sir James Hall and Mr. Gregory Watt (Phil. Trans. 1804, p. 294) have long ago 

 used the very same kind of arguments as those of Professor Bunsen ; and more 

 recently Mr. Sorby (Eeport of British Association, 1858, Trans. Sect. p. 107) 

 Hkewise showed by experiment that, when deposited from solution, crystals of a 

 very fusible substance may act as nuclei for those which are far less fusible. — 

 H. C. S. 



