DOLEEITE INTO HORNBLENDE-SCHIST. 139 



the hornblende-schist has been produced by metamorphic change in 

 the dolerite after consolidation, and it will conduce to clearness 

 if the remaining facts are described from this point of view. If my 

 conclusions should turn out to be erroneous, the phraseology adopted 

 would require modification, but the facts would of course remain 

 unaltered. 



In considering the question of metamorphism as it affects 

 the Scourie dyke, two points have to be attended to : — (1) the 

 molecular rearrangement, (2) the development of foliation. An 

 examination of a series of sections cut from different specimens of 

 the non-foliated rock shows that a gradual transition occurs from 

 the typical dolerite with lath-shaped felspar sections to a crystalline 

 granular aggregate of hornblende, felspar, quartz, and titaniferous 

 magnetic iron-ore, in which all trace of the true microstructure of 

 the dolerite has been lost. This shows that the molecular rear- 

 rangement may take place without the development of foliation. 

 The change from augite to hornblende has been so frequently 

 described that it is quite unnecessary to enter into details on the 

 present occasion. All stages of the transition may be observed in 

 the sections taken from different specimens of these rocks, but not 

 in sections taken from any one specimen. Some show the augite 

 with marginal hornblende ; others show the hornblende with a 

 distinct nucleus of augite; and others show the hcrablende con- 

 taining indistinct patches and fibres of augite. The final stage is of 

 course one in which no trace of the original augite exists. The 

 appearance of quartz in connexion with this change has been already 

 referred to. 



The changes in the felspar substance are equally remarkable, 

 though not so well understood. The original felspars, with their 

 characteristic twin-striation, may be traced into an aggregate of 

 distinct grains frequently without twinning, and having independent 

 optical relations. The general structure of these aggregates fre- 

 quently reminds one of saussurite. 



Sometimes the felspar substance is turbid, but in many sections 

 it is perfectly clear and limpid, and difficult to distinguish from the 

 quartz, which also occurs in these sections. I regret that I am 

 unable to give a more complete account of the modifications which 

 the felspars undergo. That they suffer a complete molecular rear- 

 rangement is the only point which seems tolerably clear. The 

 modification of the titaniferous magnetic iron-ore is partly mecha- 

 nical and partly molecular ; that of the apatite is, I think, simply 

 mechanical *. 



* The relations of the indi-vidual crystals and crystalline grains which form 

 the constituents of rocks, to the deforming forces brought into operation in 

 connexion with orogenetic movements, is a subject that will require much care- 

 ful investigation. One or two facts of importance appear to be established. 

 Under certain circumstances, as Prof Bonney, Dr. Lehmann, and others have 

 shown, the individual minerals become crushed, and a rock to which the term 

 clastic is strictly applicable may be produced without the agency of denudation. 

 Under other circumstances the deformation appears to be accompanied by 

 molecular rearrangement without anything analogous to crush. Thus Heim 



