Gahbroid Rocks of Minnesota. — [ [ 'inchcll. 2 2 1 
bite to anorthite. Fouque, in a detailed study especially de- 
voted to the optic properties of the feldspars of the volcanic 
rocks, furnished much very valuable data which Michel Levy 
has used in support of the theory. Fouque reached the con- 
clusion that a continuous series does not exist between albite 
and anorthite; on the contrary a certain number of well de- 
fined types exist which are capable of forming physical com- 
binations. In other words labrador-bytownite of Michel 
Levy (Ab^Ans) is not an isomorphous mixture of albite and 
anorthite, but of labrador-bytownite of Fouque (Ab3An4) and 
bytownite, which latter however, are not mixtures, but inde- 
pendent types, "forming with the other plagioclases a natural 
family comparable to the series so well established of organic 
chemistry." In such series it is still true that all the characters 
of the intermediate compounds are exactly determined if those 
of the two extremes be fully known. Thus the theory leads to 
precisely the same practical conclusions as the theory of 
Tschermak, differing from the latter mainly in assigning to 
certain intermediate compounds the same rank as pertain to 
the extremes. It is a theory based chiefly upon the frequency 
of certain types, and is very hard to attack, whether one accept 
it or not. Michel Levy has shown that slight variations seem 
to exist uncalled for by a strict interpretation of the theory of 
Tschermak, but he reached the conclusion nevertheless, that 
the best results which have been obtained showed an agree- 
ment quite satisfactory, and that if the theory is not mathemati- 
cally exact, it is as near it as any ordinary physical law. In 
any case, whether one accept the theory of a continuous series 
between albite and anorthite, or, on the contrary, admit the 
existence of a limited number of types intermediate between 
the two end terms, it is necessary for convenience of petro- 
graphic description to establish divisions in the series. 
These divisions as adopted by different writers are given in 
the following table, as far as they concern the minerals under 
discussion. It is necessary to remark at once that if one ac- 
cept the theory of Tschermak the composition of all the in- 
termediate compounds can be very conveniently represented 
by the formula, Abn An^ ; this serves in any case as a very 
simple standard of reference. 
