Kawson. I 



Mcdignite, 



35' 



Petrographic Relations. — If the rock having the mineralogical 

 characters set forth in the preceding pages were the only facies of 

 the Poohbah Lake laccolite, and the rock were considered only as 

 a mineralogical aggregate, without reference to its chemical compo- 

 sition, it would probably be classed with the nepheline-syenites. 

 Its intimate mineralogical affiliation with other important facies of 

 the same mass, which can not possibly be classed with the nepheline 

 syenites on mineralogical grounds, serves, however, to cast serious 

 doubt upon such a reference. This doubt leads rapidly to a denial 

 of the correctness of the reference the moment we inspect the 

 chemical analysis of the rock and compare it with that of repre- 

 sentative nepheline-syenites. We see at once that its low silica 

 contents, 47.85 percent, and the high lime, 14.36 per cent., are alone 

 sufficient to segregate the rock from the nepheline-syenites, heter- 

 ogeneous as that family is. The proportion of magnesia, 5.68 per 

 cent., is, moreover, quite at variance with that of the nepheline- 

 syenites. It is worthy of note, too, that the potash is in excess of 

 the soda, while the reverse is generally true of the nepheline-syenites. 

 On the other hand, the nepheline-pyroxene-malignite is chemically 

 closely akin to certain Vesuvian leucitophyres.as may be seen by an 

 inspection of the analyses given in the table. From these it chiefly 

 differs in its more moderate content of iron and alumina. It appears 

 therefore that we have in this malignite the plutonic equivalent of a 

 well-known volcanic rock — the leucitophyre of Vesuvjus. 



In addition to the broad characteristics which segregate this 

 rock from the nepheline-syenites, certain other features may be 

 alluded to, which, although from a taxonomic point of view are 

 probably of minor or varietal significance, have considerable intrin- 

 sic interest. These are the peculiar poikilitic structure of the rock, 

 the total absence of plagioclase, and the remarkably high proportion 

 of phosphoric acid revealed by the analysis. None of these is to 

 be held a distinctive or constant feature of this chemical type of 

 basic plutonic rock; but they combine to add interest to this occur- 

 rence of it, and to emphasize its unique character. It is, moreover, 

 noteworthy that in its high contents of phosphoric acid the nephe- 

 line-pyroxene-malignite is even approximately paralleled by no 

 other rocks save the Vesuvian leucitophyre, with which it is on other 

 grounds correlated. 



