732 General Notes. 
MINERALOGY AND PETROGRAPHY.' 
the Archean rocks of Missouri. In a diabase porphyrite, from 
Madison county, are areas of quartz, which seem to be the remains 
of porphyritic crystals, whose original outlines have been pene 
by the corroding action of the diabase magma before its consolida- 
tion. Since an analysis of the rock reveals a content of only 53.47 
of silica, it would seem that we have another instance illustrating 
the possibility of the separation of free silica from a very _— 
magma. In another porphyrite the rare rock constituent pieta 
tite was detected. A very instructive fact in connection giri a 
Missouri rocks is the existence of granites of idiomorphic seers 
crystals which have been enlarged by the addition of orthoe e 
material. In some cases this enlargement is in the shape ofa E ina 
ophyre border, and in others of a solid vein of irregular out 7s 
surrounding a well-marked idiomorphic crystal_—In the course 
an examination of a minette from Weiler, near Weiselburg, d 
Linck’ found the porphyritic crystals of biotite to be surroun 
1 Edited by Dr. W. S. Bayley, Colby University, Waterville, Me. 
2 Neues Jahrb. f. Minn., 1888, i., p. 117. 
s Amer. Naturalist, 1887, p. 850. 
* Johns Hopkins Univ. Circulars, No. 65, April, 1888. 
i i , . 72. thringeD, 
6 Mitth. d. Comm. f. d. Geol. Landesuntere, von Elsass-Lo 
1887, i., p. 69. 
