Haworth on the Archcsan Geology of Missouri. 365 
through the rock mass, so that it gives the rock a spotted ap- 
pearance. A portion at least, of the so-called fragmental rocks 
owe their spotted color to this cause. 
Orthoclase and flagioclase. One of these minerals is al- 
ways present and in many cases the two exist side by side. 
The presence or absence of quartz does not determine the 
relative amounts of them. In the more basic varieties the 
plagioclase is labradorite or bytownite, and in the more acid 
ones it is oligoclase or andesin, as is shown by their specific 
gravity determinations. In specimen number 273 the results 
obtained were 2.716, and in number 223 it was 2.665. It 
is interesting to note that number 223 has no porphyritic 
quartz whatever, while number 273, with the basic feldspar, has 
a great deal of quartz in the very coarse ground mass by the 
side of large quantities of epidote. Usually the feldspars are 
quite fresh. When altered they yield the ordinary products of 
alteration. 
Epidote. This mineral is present in varying amounts in a great 
many of the porphyries. Its irregular distribution in the rock- 
mass— often occurring in partially decomposed feldspars — and 
the total absence of crystalline form indicate that it is of secondary 
origin and suggest that probably it was mainly derived from an 
iron silicate which has been wholly changed so that no vestige 
of it can now be discovered. The epidote varies from the small- 
est grains to those an eight of an inch in diameter. As we pass 
south from Pilot Knob the amount gradually increases. In 
some places, particularly at ' the Piedmont quarries, it is suffi- 
ciently abundant to give the rock a spotted, dark green color. 
Piedmontite. This rare manganese epidote exists in small 
quantities in the porphyries at different places. It is most abun- 
dant at the quan-ies four miles southeast of Anapolis, but traces 
of it were seen in specimens from Piedmont; and, judging from 
the similarity of different rocks, it should be looked for at all those 
quarries along the road between Hogan and Piedmont. Speci- 
mens numbers 278 and 279 have the largest amount of the min- 
eral of any examined. They are reddish, coarse-grained porphy- 
rytes, consisting essentially of a quartz and plagioclase ground- 
mass through which u-on-oxide, epidote, and the piedmontite are 
scattered. The porphyritic constituent is plagioclase. The pied- 
