IOWA ACADEMY OP SCIENCES. 
145 
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A 
clino-pinacoid which with the unit prism and the positive hemi- 
pyramid are the only forms occurring on crystals found in this 
vicinity. The twin crystals are united along the ortho-prism. 
Shadow crystals are common both in simple and twinned forms. 
In the latter there is also an arrangement of impurities along 
the common axis of the two individuals from which barbs are 
thrown out in all directions. These 
prongs intersect with an approximate 
angle of 66 degrees which is found to 
correspond rather closely with the 
conical angle of the cone-in-cone struc- 
ture common in the vicinity of Fort 
Dodge. The resemblance between 
the structures is very striking. Meas- 
urements of simple crystals show them 
to be commonly a trifle less than twice 
as long as broad, and their thickness 
something less than half their breadth. 
In twin crystals, however, the relative 
thickness is more than doubled, mak- 
ing each individual nearly as thick as 
broad. It will be noticed that the 
growth of these crystals is in one 
direction, and that they become 
smaller and smaller by frequent re-en- 
trant angles into which fine clay has Gypsum crystal from iieaTFort 
filtrated. With the rates of growth in Dodge, 
the several directions as expressed above, it is evident that the 
angle receiving the foreign material is pushed upward approxi- 
mately twice as fast as outward by reason of which the impur- 
ities are left in a cone whose angle is in the neighborhood of 
60^ Only occasionally is one of those angles persistant, the 
majority of them being grown over and buried. 
Peculiar growths are frequently observed where a small 
crystal protrudes from the prismatic face of a much larger 
individual. The faces of the prismatic zone of the small crystal 
are replaced by vicinal planes which come together in a point 
forming a sharp six-sided pyramid. 
Cavities in the gypsum rock sometimes contain crystals of 
selenite, but the rock consists of fibrous gypsum, crystals being 
rare. 
