144 
IOWA ACADEMY OF SCIENCES. 
been noted, though careful search would doubtless reveal well 
crystallized specimens. The mineral occurs in masses dissemi- 
nated through the coal and shale with slightly projecting crys- 
tals, showing the octahedron and cube, the latter often with 
curved faces. Certain layers of the brick shales are rejected 
because of its presence. At the clay pit of the Fort Dodge 
Pressed Brick Company a good sized stigmaria impregnated 
with pyrite was found. The pyrite is associated with gypsum, 
crystals of which are often found clustered around masses of it. 
Pseudomorphs of limonite after pyrite are common in the Saint 
Louis limestone. 
Celestite. Sulphate of strontium has been noted at several 
places. It is usually in fibrous layers from a quarter to three 
inches thick in the coal measure sandstones and shales. The 
seams are variously inclined, one observed having a high angle. 
A cross-section of the vein showed two or three layers, indicat- 
ing that the fissures had been widened at different times. 
White who had evidently seen only the horizontal seams sug- 
gested that the joints were connected with the stratification of 
the inclosing shales. The same writer stated that the fibers 
are perpendicular to the plane of the layer, but samples have 
been obtained where their direction is variously inclined. An 
inspection of the material collected from the bed of the river a 
mile or so below Fort Dodge also brings out a variation in the 
angle between the long direction of the fibres and the basal 
cleavage. In those seams which are horizontal the cleavage is. 
parallel, or nearly so, to the plane of the layer and the fibers 
are at right angles to this. In the mineral from those seams 
which are most nearly vertical the cleavage is inclined both to 
the sides of the fissure and the direction of the fibers. How- 
ever the basal plane is always horizontal, suggesting that grav- 
ity has played an important part in the orientation of the crys- 
tal network. From the imperfect cleavages of the brachy- 
pinacoid it is evident that the vertical seam, the fibers, are 
parallel to a br achy -dome. Some fairly good tabular crystals 
were obtained from the clay pit of the Fort Dodge Pressed Brick 
Company, where the mineral is associated with pyrite often 
filling the interior of sigillaria. 
Girpsum. Crystals of gypsum occur in the coal measure 
shale in considerable perfection of form. So far as observed 
these crystals are invariably of tabular form parallel to the 
*Geol. of Iowa, vol. 11, p. 305. 1870. 
