1 36 The Ajnerican Geologist. September, i898 
thin bands. The whole produces a gneissic structure similar 
to that found in granites. This structure was seen particu- 
larly in the northeast corner of T. 61 N., R. 10 W. and north 
of West Greenwood lake, T. 58 N., R. 10 W. 
The preceding structures of the gabbro are primary con- 
ditions incident to the solicUfication of the gabbro magma. 
Frequent reference has been made by other writers to the 
bedded or sheeted structure of the gabbro. This has been the 
chief argument in favor of considering the gabbro mass as 
composed of separate surface flows. The structure is a fissil- 
ity produced by dynamic forces acting upon the gabbro mass 
after its solidification. The strike of the layers is nearly north- 
east and southwest, but there is a tendency to conform in di- 
rection to the outer limits of the gabbro. The prevailing, dip 
is 20 to 45 degrees to the southeast or 20 to 45 degrees to the 
northwest. Occasionally the two sets of fissility or cleavage 
are found in the same locality and the rock then breaks into 
cubes or rhombs instead of splitting into large slabs. The 
tissility traverses the rock irrespective of the direction of the 
primary structures. When these structures have the same 
strike and dip as the fissility the two coincide. A difference 
in dip of 10 degrees between these structures will make both 
visible. The fissility is common to all parts of the gabbro. 
The layers vary in thickness from one inch to ten feet. Clififs 
which possess the fissility and which have been subjected to 
extensive weathering have the appearance of a wall of ma- 
sonry. 
Texture. The size of the minerals constituting the gabbro 
varies from fine grained to very coarse grained crystals and 
crystal grains sometimes six inches across. The usual size 
is medium to coarse grained. The prevailing texture of the 
gabbro is the granitic. The other textures are developed only 
locally and seldom of great extent. The granulitic texture 
is limited to certain contact phases of the rock, and is espe- 
cially well developed along the northern edge and in the vicin- 
ity of Brule lake. It is the texture common to the fine grained 
parts of the gabbro. 
The poikilitic texture is rarely found in the relations of the 
essential minerals to each other. The accessory minerals are 
frequently enclosed within the larger grains of the essential 
minerals. 
