1894. | NEW YORK ACADEMY OF SCIENCES. 215 
an important, if not the chief, cause of the cavernous character 
of the magnetite. 
These facts lead to the conclusion that the relation between 
the magnetite, biotite and plagioclase is not accidental, but is of 
a genetic character. The tendency for biotite to surround mag- 
netite has often been remarked in rocks of this kind, and by 
many has been regarded as a simple mechanical attachment of 
the biotite to the ‘earlier formed grains of magnetite. Others, 
particularly in this country Hawes* and W adsworth,+ have 
contended that the biotite results from the interaction of the 
magnetite with the feldspar, or, more rarely with other min- 
erals. That this is true in the present instance there can be 
little doubt, for if the scales of biotite formed independently 
and attached themselves to the magnetite mechanically there is 
no reason why the biotite should not intervene between the 
magnetite and the augite, just as well as between the magnetite 
and the plagioclase, and yet this very seldom happens, although 
contacts between magnetite and augite are abundant. So con- 
stant an association as that of the magnetite, biotite and plagio- 
clase can not be fortuitous, and requires some explanation. 
The suggestion that the biotite is a reaction rim affords such an 
explanation, according at once with the facts observed, and, in 
a general way, with our knowledge of the composition of the 
minerals concerned. The details of the process are, it is true, 
obscure, for while the biotite is to a certain extent intermediate 
in composition between the other minerals, it could not be 
formed from them without the introduction of foreign materials, 
or the removal of considerable quantities of some materials 
present, or both. But this fact need not throw doubt upon the 
explanation advanced, as the same is true of most of the reac- 
tions and alterations constantly seen in rocks. Many of these 
details would doubtles be made clear by analyses of the minerals 
involved, while others might well elude all our methods of in- 
vestigation. 
Occasionally there is outside of the biotite a zone of white, 
granular substance of uncertain character. It is probably leu- 
coxene, representing an excess of titanium in the magnetite, 
which has separated in this form during the series of changes to 
which the minerals have been subjected. The biotite itself 
yields quite readily to the attack of the agents of alteration, and 
passes over into a deep green chlorite, with strong pleochroism. 
Apatite is an abundant constituent of all the dikes, occurring 
in long, slender prisms, often broken across. 
* G. W. Hawes—Geology of New Hampshire, 1878, p. 205. 
M. E. Wadsworth—Geol. and Nat. Hist. Survey, Minn., Bull. II., p. 65. 
