Wild. — Geology of the Bluff. 323 



The inference that all the hornblende in this rock is secondary i.s 

 supported by descriptions of similar areas in various parts of the 

 world. Such areas have been described by Irving* and by Williamsf 

 in America, by Phillips^ in Cornwall, by Reusch§ in Norway, by 

 Lehmann|| in Saxony, by Beckell in Lower Austria, by Wadsworth** 

 and by Hawesff in America, and more recently by HarkerfJ in the 

 west of Scotland. 



As regards the causes that produced the alteration of the pyroxene 

 little is yet known. The experiments of Mitscherlich and Berthier (1824), 

 Gustav Rose (1831), and Professors Fouge and Michel Levy, of Paris, 

 and the recent researches of Vogt, Joly, Cusack, Doelter, Brun, Day, 

 Allen, and others have shown that augite appears to be the stable form 

 at high temperatures and hornblende at low temperatures. From this it 

 may be assumed that any condition tending to facilitate molecular 

 readjustment at ordinary temperatures must necessarily tend to facilitate 

 the change from augite to hornblende. 



These considerations inclined Williams§§ to ascribe the uralitization 

 of some rocks to the action of great pressure, such as might be exerted by 

 the upheaval of mountains, and Lehrnann|| and HatchWl readied similar- 

 conclusions. Subsequently, however, Williamsf decided that, though 

 pressure may, and doubtless does in many instances, assist in the para- 

 morphism of pyroxene in rocks, it cannot in all cases be regarded as even 

 a necessary adjunct. 



In the case of the plutonic mass of rock forming Bluff Hill the follow- 

 ing points are put forward merely as suggestions. 



The magma was intruded at sufficient depth to allow of the formation 

 of a holocrystalline mass by slow cooling. At the temperature of the mass 

 augite was formed. When ordinary temperature was reached the augite 

 would tend to change to hornblende if conditions should change so as to 

 induce unstable equilibrium in the crystals so far as the molecular forces 

 were concerned. Such a change of conditions would possibly be brought 

 about by either or all of the following : — 



(1.) Diminution of pressure by denudation of the overlying rocks. 

 This undoubtedly took place, but whether it would tend to induce 

 molecular readjustment is a matter for speculation. 



(2.) Movements of depression and elevation described above. 



(3.) Lateral pressure due to the folding to which the whole country 

 was submitted in late Palaeozoic or early Mesozoic times. 



* Irving, R. ()., " Origin of the Hornblende of the Crystalline Rocks of the North- 

 western States," Am. Journ. Sci., vol. 26 (1883), p. 32. 



t Williams, G. H., " The Gabbros and Associated Hornblende Rocks occurring in 

 the Neighbourhood of Baltimore, Md.," U.S. Geol. Surv., Bull. No. 28, 1886. 



% Phillips, Quart. Journ. Geol. Soc, vol. 32 (1876), p. 155, and vol. 34 (1878), p. 471. 

 § Reusch, " Die fossilienfuhrenden krystallinen Schiefer von Bergen in Norwegen," 

 German translation by R. Baldauf, 1883, p. 35. 



|l Lehmann, " Untersuchungen iiber die Entstehung der altkrystallinischen Schiefer- 

 gcsteine," p. 190; Bonn, 1884. 



•j Becke, " Mineralogische und petrographische Mittheilungen," vol. 4, p. 357, 1882. 

 ** Wadsworth, "Bulletin Museum Comparative Zoology of Harvard College, Cam- 

 bridge," vol. 7, p. 46. 



ft Hawes, G. W., Am. Journ. Sci. (3), vol. 12, p. 136. 

 %X Harker, A., Mem. Geol. Surv., Tert. Ign. Rocks Skye (1904), p. 319. 

 §§ Williams, G. H., Am. Journ. Sci. (3\ 28, p. 266 (1884). 

 If If "Mineralogische und petrographische Mittheilungen." vol. 7. p. 83 (1885). 



11* 



