161 



connection with contact metamorphism. as well as the more common uralitic 

 and actinolitic modifications. 



Pressure or regional metamorphism also changes augite to hornblende. 

 The development of hornblende at the expense of the augite or diallage of the 

 basic igneous rocks, in regions not affected by contact metamorphism, has 

 been described by a number of observers (1) ; and may now be regarded as one 

 of the most definitely established facts in petrographical science. The 

 secondary hornblende may occur in the compact, uralitic or actinolitic 

 conditions. The compact hornblende may be either brown, green, or colourless. 

 Green varieties are perhaps the most common, and in the metamorphosed 

 gabbros these frequently assume the bright grass-green tint which 

 characterises smaragdite. The compact uralitic and actinolitic varieties 

 may occur together ; but more frequently the compact variety is not found in 

 association with the other two. The gradual replacement of diallage by 

 irregular grains of compact hornblende is well seen in some of the Lizard 

 gabbros. The change appears to commence at the margins and to extend 

 inwards until all trace of the original mineral has disappeared. Large crystals 

 of diallage, measuring an inch or more in diameter, may be converted into ail 

 aggregate of small hornblende grains without any uniform orientation. This 

 change is often accompanied by a change of the felspar to saussurite, and by 

 a plastic deformation of the rock-mass which results in the development of the 

 "flaser," "augen," and schistose structures to be subsequently described. 

 We have no information as to the relation between the secondary hornblende 

 and the original diallage in these rocks so far as chemical composition is 

 concerned. STRENG, however, has analyzed the diallage and hornblende 

 occurring under similar conditions in the Hartz gabbros. 



101-85 ... 98-15 ... 100-24 ... 99-65 ... 100-38 



3-00 3-01 



(1) STKEXG, Gabbro des Harzes, N.J., 1862, p. 933; BONNET, on the Serpentine and 

 associated rocks of the Lizard District, Q.J.G.S., vol. XXXIII. 1877, p. 884; LOSSEN, Studien 

 an metamorphischen Eruptiv- und Sedimentgesteinen. Jalir. d. Preuss. geologischen 

 Landesanstalt fur 1883 and 1884 (where references to previous papers on the same subject dating 

 as far back as 1869 will be fonnd; LEHMANN, Die Entstehung der altkystallinischen 

 Schiefergesteine. Bonn. 1884, p. 190; HATCH, TJber den Gabbro aus der Wildshonau in Tirol, 

 1885, p. 75 ; G. H. WILLIAMS. The Gabbros and associated Hornblende rocks occurring- in the 

 neighbourhood of Baltimore, Bull, U.S., Geol. Survey, Xo. 28, 188<>. 



