342 Herrick et al. on American Norytes and Gabbros. 
and Rosenbusch's statement that it has been found only in the 
northern regions of America. 
3. The adjacent paragenous rocks are hardly less interesting 
than the noryte. They consist of accessory dykes and flows of 
daj-k porphyritic dioiyte^ very dense and hard. The section 
displays much segregated biotite and hornblende with iron 
oxides. The feldsjoars are oligoclase and orthoclase with pre- 
vailing Carlsbad and occasionally Baveno twins. The feldspars 
and quartz are full of liquid and other inclusions. The most 
important solid inclusions are garnets in large, richly-colored 
specimens as well as in very small perfect isotropous crystals. 
Apatite is likewise abundant. The structure of the diorytes 
can be easily explained by reference to the schists which make 
the covmtry rock at this place. These schists abound in corun- 
dum minerals and the silicates, and there are local masses of 
rutile, but a chemical examination of the magnetite of our 
noryte failed to reveal any titanium. The magnetite in the 
diorytes is in the form of aggregates occupying elongated or 
dendritic cavities among the hornblende and mica. Hiematite 
is abundant as an alteration product. 
3. The Dulnih gabbros. This is a peculiarly interesting 
series and has already been the topic of considerable discussion. 
It is probable that the vexed question of the relations of the 
rocks associated at Duluth could be solved by a sufficiently ex- 
tended study in the field and the laborator\-. Meanwhile, a fe\v 
sviggestions are offered. 
The rock which may, perhaps, be regarded as typical of the 
primary overflow, is seen at the north east of the cit}-, "where it 
is now extensively cpiarried. It is an intensely black rock with 
a specific gravity of 2.77, containing long bladed feldspar and 
regular but corroded augite. The deep color is largely due to 
the suffusion of iron oxide, the magnetite being badl}' corroded 
and affording haematite by alteration. Technically this species 
is diabase, but it is closely related with the gabbros and posses- 
ses certain peculiarities which may explain the relation. The 
structure of the feldspar is peculiarly instructive upon this point. 
Many of the plagioclases are zonary, and, though a hasty ex- 
amination might read them oligoclase, greater care will generally 
show that the central zone is of a higher basidltv than the outer; 
