Haworth on the Archcean Geology of Missouri. 377 
to show the general outHnes of the crystal and to have two 
or more planes well developed. Much more often, however, it 
is very irregular in shape, and not unfrequently is entirely sur- 
rounded by augite. Lines of fracture running through the crys- 
tals are ver}^ common. In the unweathered specimens the olivine 
is remarkably fresh, and is almost entirely colorless. As de- 
composition progresses the borders and walls of the fracture lines 
turn green, iron-oxide separates out — usually magnetite, but 
sometimes a blood red oxide — and serpentine is obtained as the 
end product, but the amount of this last is very small. 
Hornblende. It is quite probable this mineral never occurs in 
the Missouri diabases as a primary constituent. Green, fibrous 
hornblende is found in many of the specimens, and brown horn- 
blende in three or four of them. In the greater number of in- 
stances it exists under such conditions there can be no doubt 
concerning its secondary origin. Number 335 shows this quite 
well. It is a coarse-grained rock with some of the crystals fully 
one and a half centimetres long. The borders of nearly all 
these augites are completely changed to hornblende, either green 
or brown. It is not uncommon to find such a crystal with a rim 
of brown hornblende forming but one individual. Fig. 4, pi. i, 
represents a twin augite, with a green hornblende border, the 
portion on the smaller member of the twin being a single indi- 
vidual. 
Number 370 also shows the change of augite to brown horn- 
blende. Here we have a very fresh augite with few if any 
crystallographic outlines. In some individuals there are numer- 
ous little spots in which the change is complete. In other cases 
the altered parts are much larger, and in a few instances olivine 
crystals are entirely surrounded by the hornblende which is 
probably secondarv. 
The green hornblende of the uralite-diabase is usually quite 
fibrous. It is an interesting fact that in the specimens examined 
not a single olivine-diabase is uralitized, while very few dia- 
bases free from olivine were found in which uralitc was not 
abundantly developed at the expense of the pyroxene. 
Quartz. Few of the diabases contain quartz, but number 235 
has a considerable amount of it which forms a beautiful micro- 
pegmatite with the feldspar. In other cases secondary quartz 
