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passage of a massive igneous rock into a crystalline schist. It also illus- 

 trates the passage of a massive plagioclase-pyroxene rock into a massive 

 plagioclase-hornblencle rock (epidiorite). The general course of the dyke 

 is N.W. and S.E. ; that of the gneissic banding is E.N.E., and W.S.W ; so 

 that the dyke cuts across the banding. The age of the dyke is not at 

 present known. Those portions of the dyke which have retained their 

 original characters consist essentially of plagioclase, pyroxene, and titani- 

 ferous iron-ore. The plagioclase often gives lath-shaped sections. It 

 penetrates the grains and plates of pyroxene and possesses all the charac- 

 ters of the plagioclase of doleritic rocks. The pyroxene is pale in colour, 

 and for the most part irregular in form. Sometimes it shows diallagic 

 structure and sometimes not. The inclusions are seen to be arranged in 

 a rude kind of zonal manner in some of the grains. The opaque iron-ore 

 occurs in irregular plates. The rock is of medium grain, that is, the 

 different constituents can be recognised with the naked eye or with a 

 lens of low magnifying power. Every gradation from a rock of the above 

 character (see Fig. 1, Plate XIX.), to one essentially composed of plagioclase 

 and hornblende (see Fig. 1, Plate XX.) can be observed. The gradual 

 replacement of the pyroxene by hornblende is the most striking feature. 

 The change appears to commence at the margins of the pyroxene-grains 

 and to proceed until all traces of the original mineral have disappeared 

 The resulting hornblende may be fibrous, actinolitic, or compact. It often 

 occurs as extremely ragged plates, containing detached granules of a 

 water-clear mineral having the refraction and double-refraction of quartz. 

 The forms of the original pyroxene-grains are for the most part lost, so 

 that the replacement of pyroxene by hornblende cannot be strictly de- 

 scribed as a case of either pseudomorphism or paramorphism. The 

 modification of the pyroxene is accompanied by a modification of the 

 felspar. In the epidiorite this mineral usually occurs as aggregates of 

 irregular water-clear grains, in which needles and grains of hornblende 

 occur as inclusions. In the least altered plagioclase-pyroxene rock the 

 felspar is usually more or less turbid on account of alteration or the 

 presence of minute inclusions ; in the epidiorite the felspar is remarkable 

 for its freshness. The outlines of the felspar-areas in the epidiorite do not 

 correspond with those of the felspar-areas in the original rock. The 

 micro-structure of the original rock, determined by the successive crystal- 

 lization of the different constituents from an igneous magma, has been 

 almost entirely obliterated by the subsequent changes. Rocks having the 

 structure and composition of the epidiorite (Fig. 1, Plate XX.) are by no 

 means rare in the Highlands of Scotland, and an important question arises 

 as to whether they are in all cases the result of the metamorphism of 

 plagioclase-pyroxene rocks. All that can be said with safety at present 

 is that such rocks have been proved to have been formed by the meta- 

 morphism of plagioclase-pyroxene rocks and have not been proved to 

 originate in any other way. There is no evidence to show that such rocks 

 can be produced by the consolidation of an igneous magma. The Scourie 

 dyke also furnishes evidence of the passage of a massive rock into a 



