of the Bud tan District 17 



slices the concentric and radial structure is beautifully dis- 

 played. It is very common to find these amygdules 

 bordered by a bright green granular mineral, which, in its 

 inner margin, often extends in filaments or acicular prisms 

 into the agate. This mineral polarises as an aggregate, 

 and is not, I think, dichroic. The acicular crystals border- 

 ing it suggest epidote to me. This mineral is not attacked 

 by digestion with hot sulphuric acid. I suspect that it may 

 be an aggregate of silica, coloured by some pigment. I am 

 at present unable to further define it. Finally, ferric oxide, 

 as translucent plates, and ferric hydrate (brown iron ore), are 

 found as alteration products in all parts of the rock-mass. 

 Some of the magnetite is, doubtless, also of secondary 

 origin. 



The microscopic analysis of this rock shows that its 

 original constituents have been plagioclase felspars, augite, 

 and magnetite, with apatite, and, in almost all cases, a second 

 pyroxenic mineral enstatite. 



The felspars evidently greatly predominate in amount 

 over the pyroxenic minerals. The entire absence of olivin 

 places such a rock, it being of pretertiary age, at once, and in 

 accordance with the classification I here follow, in the great 

 Diabase group.* The presence of plagioclase and augite 

 as porphyritic crystals in a ground-mass, containing more or 

 less basis, limits its range to the porphyritic forms of Diabase: 

 and the occurrence of the rhombic pyroxene enstatite again 

 restricts it to a particular section. The rock, therefore, 

 belongs, according to the microscopic examination, to the 

 enstatite-bearing section of the Diabase porphyrites. 



Having thus arrived at a conclusion, based upon micro- 

 scopic examination and optical data, it will be well to see 

 how far these are confirmed or contradicted by a quantita- 

 tive chemical analysis and a discussion of the results. 



The sample which I took for analysis was selected from 

 near Moore's Crossing. It was of a blueish-black colour, and 

 did not show any of the red tinge which in these rocks 

 indicates alteration. It had a finely crystalline structure, 

 and showed some porphyritic, but still minute, white felspar 

 crystals, as also rarely small crystalline grains of pyrite. 



I prepared several thin slices of this sample, cut perpen- 

 dicularly to each other. I found it to be typical of the 

 porphyritic rocks of the group. It was, on the whole, re- 



* Rosenbusch. — PhysiograpJiie der Mas&igen Gesteine, 1877. 



