142 



J. J. H. TEALL OlSr THE METAMOEPHO SIS OE 



One fold occurring not far from the dyke is figured below (fig. 2). 

 The parts marked with crosses are lumps of hornblende -rock. 



Fig. 2. — Fold in Hebridean Gneiss, 3 or 4 feet across. 



Another argument tending to show that the foliation has been 

 developed by mechanical action subsequent to consolidation may 

 be founded on the fact that the parallel structure does not occur in 

 the rock in which the original augite and felspar may be recognized. 

 The hornblende occurs alone in the foliated portion. 



The relations of augite to hornblende are of great interest in 

 connexion with the origin of the crystalline schists, and they have 

 recently been discussed by the American lithologists, Prof. E. D. 

 Irving * and Mr. Gr. H. Williams t, at considerable length. Mr. 

 Williams points out that augite appears to be the stable form at 

 high temperatures, hornblende at low temperatures, so that any 

 condition tending to facilitate molecular readjustment at ordinary 

 temperatures must necessarily tend to facilitate the change from 

 augite to hornblende. The enormous pressures brought into 

 operation in the process of mountain-making may, as Mr. Williams 

 states, not unreasonably be supposed to supply such conditions. 

 The relation of crystals to their environment is one which lithologists 

 have most carefidly to consider. A crystal is doubtless in stable 

 equilibrium, so far as the molecular forces are concerned, when subject 

 to the conditions under which it is formed. Under other conditions 

 it may be in unstable equilibrium, and ready therefore to undergo 

 molecular readjustment the moment such readjustment becomes 

 possible. 



We conclude, then, (1) that the hornblende-schist of the Scourie 

 dykes has been developed from a dolerite by causes operating after 

 the consolidation of the dolerite, and that the metamorphosis has 

 been accompanied by a molecular rearrangement of the augite 

 and felspar ; and (2) that the molecular rearrangement has, in 

 certain cases, taken place without the development of foliation. 



* Amer. Journ, Sci. ser. 3, vol. xxyi. p. 27 (1883) ; ihid. vol. sxvii. p. 130 

 (1884). 



t Amer. Joiirn. Sci. ser. 3, vol. xxviii. p. 259 (1884). 



