OF ARTS AND SCIENCES. 263 



subject to com2-)lete alteration, even when its neighbors remain wholly- 

 intact. 



Where the interstices between the constituent crystalline minerals 

 are not occupied by augite, they are filled with a transparent substance, 

 in places colorless, in others of faintest green, almost colorless, only- 

 just distinguishable from tlie colorless feldspar in ordinary transmitted 

 liglit. It often penetrates the felds|)ar crystals in cross fractures. 

 Between crossed nicols it is black, sprinkled with minute blue-gray 

 clouds, — an aggregate polarization due, probably, to either an ex- 

 ceedingly minute radiating fibrous, or a granular, structure. Under a 

 ■^\ inch objective (Hartnack's, No. 10), portions of the black seem to 

 remain dark on revolving between crossed nicols. The substance is 

 at the most only very feebly double refracting. Unfortunately I was 

 able to find with a low power, and to try with the needle on an uncov- 

 ered section, only those portions which were faintly green, and these 

 were soft. Still, I am forced to believe that we have here to do with 

 remnants of glass base, which is altered in the faint green portions to 

 a cliloritic or serpentine substance. Its whole mode of occurrence in 

 this very fresh rock shows conclusively that it is not an alteration 

 product of any of the constituent crystalline minerals. 



The relative ages of the constituents of this rock appear to be well 

 defined as follows : — 



(I.) ORIGINAL MAGMA. 



(II.) 1. Olivine. 2. Pl.^gioclase. 3. Magnitite. 4. Augite. 5. Residuary 

 I (Aiioitbite.) Magma. 



1 a. Dichroitic alter. | 



product. 5 a. Alteration 



product. 



The residuary magma (5) must have been the last to solidify ; to its 

 presence was due the internal mobility of the mass, which rendered it 

 possible for the augite to crystallize in larger individuals, and, in doing 

 so, to crowd from its centres the olivine and magnetite individuals. 



This residuary base, probably, differs in chemical constitution from 

 the original magma, since it is only the residue of this after the removal 

 of the ingredients forming the crystalline constituents. 



Highly altered Melaphyre. — While- the type represented by " the 

 greenstone " is one of the most constantly recurring varieties throughout 

 the copper district, it is very rarely iu as unaltered a condition, and it is 



