Herrick et at. on American Norytes and Gabbros. 343 
indeed, it not unfrequentl}' happens that the central portion re- 
tains its original character with multiple t^vins and a ratio of 
<: to C agreeing with labradorite (in one case 25° — 26° )while 
the outer zone is monoclinic, with characters of orthoclase. 
These zones are commonly separated by an iiTegular band of 
alteration, which is granular, greenish-white and isotropous. 
All stages between crystals in which the basidity of the zones is 
nearly the same and those described can be traced. The only 
■other characteristic of moment is the enormous development of 
apatite, (see fig. 8). (Irving remarks upon its absence in this 
series). The most obvious explanation of these structural pe- 
culiarities is that the rock was primarily a true diabase with 
labradorite as its feldspar, but that the process of decomposition 
has been inaugurated by a leeching out of the calcium and sodi- 
um and its replacement about the periphery by jDOtassium. 
The source of the potassium might be sought in meteoric water 
from kaolinizing adjacent acid rocks. But we shall have oc- 
casion below to assume for some of the adjacent orthoclase gab- 
bro a mixture of acid and basic rock on a large scale in order to 
•explain the origin of the orthoclase. 
The ordinajy gabbro of Duluth is already known from many 
descriptions. It is found exposed along the principal street and 
is used extensively for monuments, etc. The labradorite is in 
beautiful large crystals, the pyroxene is green diallage in small 
amount, magnetite and apatite making up the section. Its spe- 
cific gravity is 2.9. To the westward rise parallel ridges ex- 
tending north and south and these are composed of different 
phases of the same series. Upon the first low range appears 
the red flne-grained vai-iety next to be described, which abuts 
upon the gabbro by a tortuous but rather distinct line of contact. 
This has been called orthoclase gabbro but its relation to the 
typical sjjecies has not been explained. The hand sample re- 
sembles a close-grained hornblendic granite or syenite, but 
•obviously differs in the slender rod-like form of its feldspar. 
The section shows all the ingredients to be altered. The alter- 
ation is of the character which can best be explained as the 
result of intense heat, and resembles that induced in a granite 
which has been broken up and its fragments encased in a basic 
eruptive. There is a well-defined pseudo-porphyritic structure. 
