in the hypersthenes of the gabbros. The cnstatites of the 

 peridotites usually remaiu unattacked while the surround- 

 ing olivine is completely altered. In one rock, however, 

 enstatite was observed to pass into a fibrous brown pleo- 

 chroic mineral of high birefringence and straight extinction. 

 This is probably anthophyllite. In the same slide the 

 diopside was unaltered. 



Olivine forms ovoid grains with little or no approach to 

 idiomorphism. Its cleavage is sometimes fairly well 

 developed. It is quite colourless under the microscope. 

 It seems to be free from the schiller-like dendriform inclu- 

 sions of magnetite described by Judd 1 and Harker. 2 



Alterations.— Olivine has far more varied forms of alter- 

 ation than any other mineral in the Dundas rocks, and 

 crystals of olivine in close continuity to each other maybe 

 altered in completely different ways. Like the other 

 minerals of these rocks, its first method of alteration is to 

 become crossed by bands containing irregular depositions 

 of opaque oxides not of dendritic form. These bands form 

 most readily in the cracks of the olivine, the easiest passage 

 for solutions, though they are by no means confined to these 

 cracks. The normal alteration to serpentine, pale yellow, 

 grey-blue, or colourless takes place also, the fibres 

 developing normally to the cracks and when fully developed 

 giving a serpentine rock with the ordinary mesh structure. 



A third alteration is to calcite, siderite, and magnesite, 

 occurring in dusty cloud-like bands often crossing the grains 

 in arcuate lines, or in series of V-shaped angles, or forming 

 in massive crystals. Carbonates also occur occupying the 

 centre of a mesh in the serpentines. 



A fourth, and the most common type of alteration, is to 

 greenish silicates. Several minerals are to be found among 



