IOWA acadp:my of sciences. 33 



NOTES ON MISSOURI MINERALS. 



BV EKASMUS HAWORTII. 



1. Mehmite in a Basic Dike Bock. 

 II. Linionile PesitdoiHorpJiOHs after Cahile. 



(Fublished with foiirient of tlie Stat"' Geologist of Missouri.) 

 I. 



At various places in the Arcluwin areas of Missonn the granites and porphyries 

 are cut by dikes of ba^ic rocks which usually trend N. E. and S. W. but occasion- 

 ally in other directions. The dike rocks are comparatively constant in composi- 

 tion. When holo-crystalline they are a diabase, or an olivine diabase, and when 

 less perfectly crystallized they ^'enerally correspond to diabase porphyrite. 



Usually there are no marked indications of contact raetamorphism, either in the 

 wall rock or the dyke rock. la Sec. 1(5, T. oo, N. R. {I, E., on the East Fork of 

 Black River, in Reynolds county, is an important exception to the above state- 

 ment. On the left bank of the stream, at a point where it makes a short turn 

 from east to south, just above a small cataract, locally called The Falls, is a large 

 dike trending N. W. and S. E. which foruis the bank of the stream for a few 

 yards. At this place the dike rock rises in the form of a bluff ten metres high or 

 more, filled with vertical fissures, and presenting m every respect the appearance 

 of an eruptive rock. 



In some places along the base of this bluff the contact between the dike rock and 

 the quartz porpyhry through which the eruption occurred is quite plainly shown. 

 It seems that the lava has overflowed the walls of the fissure and is here resting 

 on top of the quartz porphyry. This dike rock is a good example of what was 

 formerly called a "green stone." Its specific gravity is 2.74o4. A determination 

 of its acidity by the St. Louis Sampling and Testing Works for the Missouri 

 Geological Survey showed that it had 4."j.40 per cent Si 0-z. Macroscopically it 

 seems to be perfectly compact excepting an occasional gas cavity now filled with 

 calcite and epidote. The freshest specimens obtainable were somewhat altered 

 by weathering, so that the hammer marks on them were ashy white. 



Microscopically it is seen that there is a considerable amount of glass present, 

 with small tnclinic feldspar crystals and much green fibrous hornblende, probably 

 secondary in origin. No pyroxene or olivine was seen in the thin sections ex- 

 amined, although it is quite possible one or both of these minerals was originally 

 present, and has been altered into the fibrous hornblende. 



In a few places along the contact line between the dike rock and quartz por- 

 phyry the dike rock has been corroded apparently by water or gas, probably by a 

 fumerole action at the time of eruption. The corrosion is not very extensive, 

 perhaps never exceeding a metere vertically. The action was sufficiently vigorous 

 to give to the rock an irregular, porous appearance. The cavities thus produced 

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