720 The American Naturalist. [August, 
differences by the extinction of the envelope in zones whose angle of 
extinction varies gradually and progressively from the centre outward, 
reaching finally (in some cases) the albite limit. This enlargement is 
regarded by the author as having taken place after the solidification of 
the rock, and at the expense of the glassy matrix. Further, the 
granophyric structure is supposed to owe its origin to a similar set of 
phenomena, viz., the secondary devitrification of a glassy matrix. 
A third very interesting article by the same writer ^ treats of the pro- 
cesses by which the plagioclase of the Oedegaarden " gepleckter-gabbro" 
has been changed into scapolite. In the fresh rock a labradorite with 
twinning lamellae is distinctly observed. Along these twinning bands 
are accumulations ^ of cavities containing solutions of sodium chloride. 
As the rock loses its granitic structure and becomes schistose the feld- 
spar loses its distinctive features, becomes granulated, and changes 
gradually into scapolite, at the same time losing its store of sodium- 
chloride solution. The production of the cavities with their contents 
of sodium-chloride is supposed to be the result of statical pressure — 
the solution having penetrated the mineral along its planes of easiest 
solution. Under the influence of mechanical stress the mineral was 
crushed and suffered granulation, reactions were set up between the 
feldspar molecules and the included sodium-chloride solution, resulting 
in entire conversion of the plagioclase into scapolite. The augite of 
the same rock presents a parallel series of changes. It first becomes 
schillerized, and then, by mutual reactions between the augite sub- 
stance and the material producing the schillerization, is changed into 
hornblende. It is pointed out by the author that similar changes 
must have taken place in the Canadian scapolite rocks studied by 
Messrs. Adams and Lawson.'" Prof. Judd would call the first kind of 
change '' statical metamorphism,'' ?,\nc& the production of secondary 
cavities in minerals and schillerization are the result of solutions 
acting on mineral substances under the influence of heat and pressure. 
"Dynamical metamorphism," on the other hand, necessitates move- 
ment in the mass, with the accompaniment of the crushing of min- 
erals and the production of schistosity. A comparison of the effects 
of the two kinds of metamorphism is briefly given by the author in 
