894 MR JOHN S. FLETT ON 



If this rock be compared with the alnoite of St Anne, Montreal, it will be found 

 that the chief differences are that it is less coarsely crystalline and much more decom- 

 posed. The olivine of the Canadian rock is mostly fresh, there is more porphyritic 

 augite ; the melilite is in larger grains, and far better preserved ; otherwise the two 

 rocks present a close parallel. 



The sequence of crystallisation appears to have been perofskite, apatite, magnetite ; 

 olivine, first augite (later corroded), biotite, and before its completion, second augite, 

 melilite, and then the brown rim of biotite ; lastly, completion of augite of ground- 

 mass, and the bulk of the melilite, with perhaps, finally nepheline or a glassy base. 



Towards the edge the dyke becomes finer grained, but the specimens are too decom- 

 posed for minute study. Numerous phenocrysts of olivine entirely altered into 

 serpentine and calcite, with many porphyritic augites still fresh, are scattered in a fine 

 grained dark groundmass. The augites are sharply idiomorphic, with a pale or colourless 

 corroded centre and a purplish margin, and in polarised light have the remarkable 

 zonal and hour-glass structure of those of the Montreal rock. The centre and periphery 

 may differ 12° in extinction, and the marked dispersion of the bisectrices observed by 

 Adams is evident here also. Biotite at the extreme edge of the dyke is present only in 

 small scales, and increases in size and abundance as we approach the centre. It is not 

 commonly idiomorphic. Melilite is not found at the margin of the dyke, but there is 

 much of a granular turbid semi-opaque groundmass which was probably originally 

 glassy. We remark here a curtailment of both the first and second generation of 

 crystals. A few grains of melanite with broad dark corroded borders were noted, and 

 here and there pale ocelli. 



In the shore at the mouth of the burns of Rennibuster, as already described, three 

 dykes are seen traversing the course of the camptonites. They are narrow, and run 

 parallel to one another, and in all probability are subdivisions of one dyke ; indeed, 

 connecting veins were observed in the flagstones between. They are all alnoites, but 

 vary somewhat in microscopic appearance. Phenocrysts of olivine occur in all, and 

 those of augite are also numerous. They resemble those described in the previous case, 

 and some contain a honeycombed centre which gives the impression that it has been 

 full of glass cavities. The corroded interior has sometimes a diallage lamination 

 parallel to the face 100. Biotite, in large irregular plates, is abundant in sections of 

 the west dyke. It is not surrounded by a darker border, and is often attached to the 

 surface of the porphyritic augite and olivine, and encloses the augites of the ground- 

 mass. It is not idiomorphic, and its period of crystallisation appears to have been 

 deferred till the formation of the groundmass. Melilite is to be found in the ground- 

 mass often with typical rectangular sections, but weathering at the edges to fibrous 

 zeolites, which lie parallel to the axis of the section and extinguish straight. In the 

 centre of these, grains of fresh material may still be found. Elsewhere the ground- 

 mass consists of secondary products, chiefly calcite, zeolites, and analcite in irregular 

 grains. The cleavage of the melilite is evident, but there is no peg-structure. In^the 



