Gabbroid Rocks of Mitinesota. — Wmchell. 155 
ly to the summit of the Mesabi range, which, at a distance of 
about fifteen miles from the lake, reaches an altitude of a 
thousand to fifteen hundred feet above the great inland water. 
Once the summit of the Mesabi hills is reached, the surface, 
while very uneven, and even rising to the lower Vermilion 
range, descends, in general, very slowly towards the north- 
west. The backbone of the Mesabi range is composed of 
gabbro — the normal olivine gabbro, rather rich in alkaline- 
earth elements, which is the central typical rock of our study. 
Its general location and wide extqnt are shown on the ac- 
companying map. 
' But with this dominant type occur variations and modifi- 
cations. Thus in many places along the northern edge of the 
gabbro area, the rock has been profoundly affected by meta- 
morphism, and it is from this region that the cordierite noryte, 
quartz gabbro, and silicoferrolyte are derived. Plagioclasytes 
(anorthosytes*) and diabases occur occasionally in the gabbro 
area itself. The troctolyte to be studied came from an area 
near Duluth, but several other areas of this rock are known 
further north, surroimded by gabbro. The orthoclase gabbro 
also came from Duluth, where it forms the country rock; it is 
also known at various other localities in Minnesota and Wis- 
consin. 
The general shape of the area occupied by these rocks is 
rudely that of a crescent, with one extremity near Greenwood 
lake pointing eastward, and the other near Duluth pointing 
southward. The extreme length of the crescent is about one 
hundred and twenty-five miles, and the maximum breadth is 
nearly twenty-five miles. The total area of this crescent of 
gabbro rocks amounts to about two thousand square miles. 
But outside of this crescent occur two smaller areas of diabasic 
rocks, and various outlying masses of gabbros and plagio- 
clasytes. The first of these areas extends west and southwest 
from Pigeon point for about forty-five miles, covering a sur- 
face of about one hundred and fifty square miles. From the 
eastern extremity of this area was obtained the type of olivine 
*The term anor*.hosyte has produced so much confusion that it 
can well be replaced by the name plagioclasyte, proposed by the French 
Commission of rock nomenclature. The latter will be used through- 
out this article. 
