of 'the Buchan District. 19 



It is interesting, where practicable, to ascertain by calcu- 

 lation the approximate proportions in which the various 

 minerals exist in the rocks examined. This may often be 

 done with considerable certainty when the chemical analysis 

 is supplemented by a microscopic examination, and more 

 especially when the rock is crystalline or crystalline-granu- 

 lar. Owing, however, to the porphyritic character of this 

 Diabase porphyrite, it is not easy to. satisfactorily calculate 

 the percentages of all the rock-forming minerals. A diffi- 

 culty arises at once in regard to the porphyritic felspars and 

 to the microscopic felspars in the ground-mass. Another 

 difficulty also arises as to the constitution of the basis, and 

 also of some of the alteration products. 



The basis being small in amount, and probably of the 

 same general constitution as the whole rock, though, perhaps, 

 more acid, I have necessarily disregarded. I have also left 

 out of view the slight traces of chloritic material associated 

 with the altered enstatite. If the rhombic pyroxene, as I 

 conclude from the microscopic examination, has been 

 almost entirety converted into bastite, then, for the latter, 

 the formula for serpentine will probably be applicable. The 

 other alterations — calcite, ferric hydrate, and agate — offer no 

 difficulties. On this basis I have attempted the calculation 

 of the analysis into the rock-forming minerals. 



The microscopic examination has shown that the augite 

 is probably a magnesia-lime augite — in other words, diopsid, 

 and, therefore, having the empirical formula (RO, Si0 2 ) where 

 EO may be (FeO, MnO, CaO, or MgO). The rhombic enstatite 

 would be the same. Not being, therefore, aluminous pyrox- 

 enes, it is not necessary to allot any of the alkalis to them. 

 There remain, then, only the felspars and the basis to which 

 K 2 and Na 2 are to be assigned; and I have already stated 

 that I leave the basis out of view. Disregarding this, and 

 the traces of viridite, the whole of the alkalis may be taken 

 to calculate the potassa and soda felspars. The amount of 

 lime felspar is given by the remainder of Al 2 3 after 

 providing for the potassa and soda felspars. The H 2 O 

 would prohably be shared between the serpentinised 

 enstatite and the ferric hydrate (limnite). The C0 2 and 

 P 2 5 give, of course, the amount of calcite and apatite re- 

 spectively. The balance of CaO and the whole of Mg.O, with 

 an equivalent number of molecules of Si0 2 , indicate the 

 augite ; and there should be some Si0 2 over as representing 

 the small agate amygdules discovered in the miscroscopic 



