226 • REV. J. MILNE CURRAN. 



lite of our basalt, = 1*92 per cent. The total soda as shown by the 

 analysis, equals 7 '34%, so that there is still 5*42 per cent, of soda 

 to be allotted to other minerals in the rock. Even if the felspars 

 are soda-lime plagioclase, they cannot use up 5*42 per cent, of 

 soda, for the microscope shows that these felspars are not particu- 

 larly plentiful in the rock ; on the contrary, the use of a quartz 

 plate shows a considerable amount of glassy base. It is not 

 unreasonable to suppose that some soda unaccounted for may exist 

 as a constituent of the residual glassy base. I may add that there 

 is nothing in the appearance of the augites to connect them with 

 the soda-pyroxenes. It only remains to decide whether the soda 

 may not be due to analcime, a soda-bearing mineral common in 

 basalts. After a persistent search I failed to find analcime in 

 any of the slices or portions of the rock selected for analysis. The 

 slides all show a well preserved rock with the olivines fresh but 

 broken and cracked, and only the smaller crystals are altered to 

 serpentine. Analcime may be present in more decomposed portions 

 of the mass ; my samples were taken from very fresh looking rock, 

 showing in thin slices nothing that can be identified as a zeolite. 

 Besides, the presence of chlorine supports the view I have already 

 taken. 



Microscopic Structure. 

 The general structure of the rock under the microscope may be 

 described as micro-porphyritic, with practically no traces of flow 

 in the disposition of the felspars. The amount of base present is 

 somewhat in excess. Much of the latter is isotropic, or so feebly 

 double refracting as to be undeterminable. The minerals which 

 occur micro-porphyritically in the base are, olivine, augite, plagio- 

 clase, aggregates of magnetite and sodalite. The olivine crystals 

 are abundant on every slide, and only occasionally showing alter- 

 ation into serpentine. The large individuals are idiomorphic, but 

 rarely perfect; they are for the most part broken and the parts 

 separated. Cubes of magnetite are common as inclusions. The 

 peculiar way wedge-shaped masses penetrate the olivine crystals 

 is really characteristic of all the slices made. 



