Editorial Covimevt. 57 
purely hypofhetical mixture, such as constitutos tlio basis of Tsclier- 
mak's theory. These associations are visible under the microscope, 
sometimes even to the naked eye. Chemical tests shdw them, as well 
as separation by heavy liquids, in all favorable conditions. It is a mat- 
ter of real phenomena and not a conception of the imaj^ination. The 
zoned crystals are exceedingly common, but a multitude of observations 
have shown their inteirral components. 
The idea of these miiu'ralojrical types is not new. Mineralouists of 
the first half of our centurj' made labradorite, oliiiocJasc and andesinc 
distinct species. Later M. Des Cloiseau.\ reco.ynized in the oligoclase 
group separate types with well-marked characters. 
How does it happen that, instead of trying to separate and define 
these types and to establish their individuality, modei'ii science has pre- 
ferred, on the contrary, to make them disappear, and rven to deny 
their actuality? ****** 
Mr. Fouqiie concludes as follows : 
1. There are feldspathic types, of definite composition, intermediate 
between albite and anorthite. 
2. These feldspars are capable of uniting together in physical associ- 
ation. 
.'{. Several of them are visible generally in the same rock, sometimes 
in large crystals and sometimes in the form of microlites, but nearly al- 
ways with the preponderance of one of them at eacli consolidation. 
4. Jlost frequently the order (jf acidity is in inver.se order of their 
formation, and the glassy material wdiich constitutes the residue after 
crystallization is more siliceous than the most acid of the feldspars. 
These aphorisms, which were announced by me some years ago, after 
the stud\' of rocks upon a limited district and with limited means of 
investigation, are to-day supported by the results of study over a large 
field, and with the aid of the perfect methods introduced during the 
last fifteen years of the science. 
The last contribution to this research is b}^ Michel-Levy, 
in a communication read before the Societe Francaise de 
Mineralogie* It is based on the spherical projections and 
figures of extinction which he had before deduced for the 
plagioclases [Etudes sur la determination des feldsjiatJis). Bj' 
means of these he constructs a general plan showing the spher- 
ical projections of simultaneous extinctions of the parts of 
compound mineral bodies, like the plagioclases, and their re- 
lations to the spherical traces of the optic axes of the com- 
pound mineral. Each of the points of these curves where the 
ellipses of the components have their axescrossed corresponds 
*Rechorche des a.ves optitjues dans un mineral [Kiuvant el re considere 
comme un melange de deux mineraux determinos. Api)lication au.v 
[)lagioclases el a la verification de la loi de Tschermak. 14 March, ISUo. 
