236 The American Geologist. April, 1902. 
mineralogic character of formations which they cover uncon- 
formably. 
Unquestionably, the interest of the geology of the Antwerp- 
Fowler mines centers in the nature and origin of the chloritic 
rock and its true relations to the ores. On returning to Boston 
with my collection, I found that in the previous year (1894) 
Prof. C. H. Smyth* had expressed the opinion, based upon 
microscopic and chemical examinations, that the chloritic rock 
is a highly altered phase of the granite. The fact upon which 
he especially relies is the occurrence in the chloritic rock of 
glassy grains and masses of quartz, which are regarded as re- 
sidual. His analysis of a specimen free from visible quartz 
shows: SiO2, 30 per cent.; MgO, 11 per cent.; FeO, 27 per 
cent.; and H2O, 12 per cent.; the remaining 20 per cent. being, 
presumably, chiefly Al2O3. This is approximately the com- 
position of prochlorite, and indicates an ultra-basic rock, while 
the granite, as represented by my specimens, is rich in quartz 
and poor in ferro-magnesian constituents, and certainly con- 
tains as much as 70, if not 75, per cent. of silica. 
The general aspect of the rock is precisely that of many 
highly chloritized traps in the vicinity of Boston; and this re- 
semblance is hightened by the slickensides by which it is min- 
utely and almost indefinitely subdivided. These testify to dif- 
ferential movement throughout the mass, and can have, appar- 
ently, but one explanation—expansion due to chloritization 
and hydration. It is manifestly impossible to find in this light- 
colored, acid granite more than a small fraction of the magnesia 
and ferrous oxide shown by Prof. Smyth's analysis of the chlor- 
itic rock. They are clearly not chiefly residual constituents of 
the granite ; and if this hypothesis stands, the main part of each 
must be imported. The only alternative is to suppose that the 
silica and alumina have suffered improbable diminution, in 
which case the expansion of the rock essential to the explana- 
tion of the slickensides would be unexplained. But where can 
we find an adequate external source of the MgO and FeQ? 
Prof. Smyth suggests the Archaean limestone and the action 
upon it of solutions derived from the oxidation of some near-by 
deposit of pyrite. But no sulphides are known in the vicinity, 
* Bull. Geol. Soc. Amer , 6, 4. 
