37^ Hatvorth on the ^rchcean Geology of Missouri. 
slender crystals which generally show the twinning lamellae very 
perfectly. A determination of its specific gravity in specimen 
346, an olivine diabase, showed it to vary from 2.671 to 2.718. 
The same kind of a determination for number 415 gave a varia- 
tion from 2.655- to 2.709. In this case the presence of a glass- 
somewhat interfered with the separation, so that the results may 
not be exactly reliable. Quite a number of extinction angles 
were measured in other specimens by the Pumpelly-L^vy 
method with the following results: 
Number 301 gave 38° 4. 37° Number 310 gave 38° 4. 36° . 
238 " 38° + 36°. 
The results of these two investigations indicate that we have 
a lime-soda feldspar varying from andesin to bytownite, with 
possibly a little anorthite. 
The size of the plagioclase individuals varies greatly. In the 
more coarse-grained rocks they can readily be seen macroscopi- 
cally, but in some of the diabase porphyrytes they are not more 
than a tenth of a millimetre long. In a few cases the other 
constituents decrease until the rock is nearly all plagioclase. 
Numbers 349 and 365 are examples of this. 
In the olivine diabase the feldspars are remarkably well pre- 
served. Occasionally rifts across the crystals have their walls 
coated with a green decomposition product, but in many of the 
rocks this cannot be seen. In the uralite diabases, and those 
badly weathered, the feldspars are often clouded with decom- 
position products. 
In number 235 orthoclase may replace a portion of the pla- 
gioclase, but the feldspars are so clouded one cannot say to a 
certainty. 
Atigite. This mineral generally occurs in allotriomorphous 
masses filling the spaces between the feldspars. In a few of the 
porphyrltic rocks crystals with well formed outlines were ob- 
served. In the fine-grained varieties the augite is scattered 
throughout the groundmass. 
In the olivine diabases it is unusually fresh, but in many of 
the others it is badly altered, sometimes to hornblende, and 
sometimes to chlorite and other products. 
Olivine. This occurs in large quantities in nearly half the 
specimens examined. Occasionally it is sufiiciently idiomorphic 
