On the Norian or " Upper Laurentian " Formation. 297 



minute inclusions above described. While one can observe 

 the cataclascic structure in the anorthosite in other parts 

 of the area, here there is scarcely a sign of pressure. 

 Broken individuals were never observed, and the feldspar 

 showed only in a few sections an occasional irregular 

 extinction. In most of the sections no trace of pressure 

 is discoverable. It is, moreover, 12 miles from the nearest 

 contact with the surrounding gneiss. The zones around the 

 olivine are very wide and perfectly developed. The olivine 

 seldom shows approximate crystal forms ; it either occurs 

 in single individuals or in aggregates, which in that case, 

 form larger grains. A single individual forms at times a 

 very irregular elongated strip. The olivine crystallized 

 before the plagioclase and became enclosed in the latter. 

 Notwithstanding that a considerable number of thin sec- 

 tions were examined, the two minerals were never found 

 directly in contact, every grain of olivine being invariably 

 completely surrounded by a double zone of other silicates 

 and thereby separated from the plagioclase. 



The first zone around the olivine is colourless, or nearly 

 so, but often shows a weak pleochroism in green and red 

 colours. It is formed of many small individuals which 

 are closely grown together, and are elongated in a direc- 

 tion at right angle to the surface of the olivine. It often 

 shows the two sets of cleavages crossing at right angles, 

 which are characteristic of pyroxene, and in sections, per- 

 pendicular to an optic axis, the revolving bar of a biaxial 

 crystal is seen. 



The individuals being so small and the cleavage is very 

 imperfect, it is very dificult to determine accurately the 

 character of this pyroxene. Similar zones, however, are 

 found in hand-specimens from other parts of the area in 

 which the crystals of the inner zone are developed on a 

 larger scale. In these the parallel extinction, trichroism 

 of red, green and yellowish colours, and also the other 

 optical properties point to a rhombic pyroxene, which 



