[HAR\^E] PALAEOZOIC BRECCIA OF THE VICINITY OF MONTREAL 261 



colour and somewhat decomposed to green chlorite. The olivine is de- 

 duced from the presence of outlines filled with serpentine and bastite. 

 Perovskite is plentiful in dusty rhombohedral outlines, being associated 

 with shadowy lath-shaped forms of calcite, which show the pegged-in 

 structure of melilite. The groundmass is doubtful hydronephelite. 



The dyke mentioned above as cutting the breccia and considered to 

 be a second stage of the intrusion is also found to be an alnoite. It 

 weathers to a light gray, but on fresh surfaces shows a dark gray 

 coloured paste, holding black phenocrysts of olivine, pyroxene and biotite. 

 In thin sections the rock is seen to be composed of the following minerals, 

 crystallising in the order given, — phenocrysts of olivine, pyroxene and 

 biotite in a groundmass of melilite, perovskite, iron ore, apatite, biotite, 

 garnet and nepheline. The olivine occurs in plentiful fresh phenocrysts, 

 some of which are extremely large. Some individuals have a perfect 

 crystalline outline, but most of them show very active magmatic cor- 

 rosion, this having gone so far in one case as to eat through the crystal, 

 leaving three fragments in a row, all giving simultaneous eitinction. 

 The olivines also show very pronounced strain shadows. The pyroxene 

 is much decomposed, but seems to have had good forms, in two varieties, 

 — augite, extinction 45°, a pale variety 30°. It is much less abundant 

 than the olivine, of the phenocrysts about three-quarters being olivines. 

 Biotite phenocrysts are rare, a few being seen in the hand specimens, 

 but none in the thin sections. Melilite is very variable in quantity, the 

 specimens although not collected with any great care, seem to show a 

 maximum proportion in the middle of the dyke and a minimum on the 

 edges, the proportion of garnet being in the reverse order. Tornebohm,^ 

 considers that in the original occurrence of alnoite at Alno, in Sweden, 

 the melilite is in part changed over to garnet. Berwerth, however, be- 

 lieves that the melilite may have been derived from the garnet.^ 



In some slides the melilite amounts to one-quarter of the field, in 

 others it is completely absent. It occurs in slender laths up to 0.46 mm. 

 long. (See fig. 4.) In general it is much decomposed to calcite, but 

 some individuals are wonderfully fresh showing parallel extinction and 

 deep purplish-blue polarisation. The melilite forms remarkable wreaths 

 or coronas around the previously crystallised olivines and augites, and 

 it everywhere has a flow stnicture. (See fig. 5.) Hogbom in his exami- 

 nation of the rock from Alno, observed that in only one instance the 

 olivine was surrounded by a melilite wreath.i Iron ore, probably ilmenite. 

 is abundant and has as its associate, perovskite in numerous rhombo- 



1 Rosenbusch, " Mikroskopische Physiographie," 1907, Vol. II, p. 706. 



^ Ibid., p. 707. 



1 Rosenbusch, op. cit., p. 706. 



