Haworth on the Archcean Geology of Missouri. 367 
give it as a manganese free epidote. But in Prof. Heddles" 
complete analysis of the mineral he finds 0.138 per cent, of MnO. 
I did not succeed in isolating a sufficient amount of the Missouri 
mineral for a quantitative analysis, but judging from the results 
of the qualitative examinations there is little chance to doubt the 
presence of manganese in sufficient quantity to make the mineral 
a piedmontite. The occurrence of this rare mineral in Mis- 
souri is quite interesting. So far as known to the writer it has 
thus far been reported from but three different localities; 
St. Marcel in Piedmont, Ital}-; Jacobsberg in Wermland, 
Sweden ; and from Japan. The last named locality was recently 
reported by Prof. Koto' who describes it as occurring very 
abundantly in the Archaean schists and gneisses. 
(c) Structure of the porphyries. 
The structure of the groundmass has always been regarded 
as a matter of first importance by students of jDorphyritic rocks. 
This groundmass may be holo-crystalline, semi-crystalline, or 
glassy. Rosenbusch divides the first of these into the micro- 
granitic and granophyr'c and designates the second and third 
respectively by the terms felsophyric and vitrophyric. In the 
Missouri porphyries a third class of the holo-crystalline ground- 
mass is so w^ell represented that it should receive special notice. 
Before describing it, however, a short discussion of the different 
possibilities in a holocrystalline groundmass may be of interest. 
The following is taken from the lecture notes of Prof. G. H. 
Williams, who kindly consented to my copying it: 
"A holocrystalline groundmass contains no amorphous or unindivid- 
ualized matter whatever, and independently of differences occasioned 
by variations in the fineness of grain, three quite distinct types of holo- 
crystalline structure are distinguishable. These three types are condi- 
tioned by the mutual relation of the quartz and feldspar crystals which 
compose the groundmass. In the first place they may be wholly inde- 
pendent, thus giving rise to a granular aggregate which is well designated 
by the term Alicrogranitic Structure. 
In the second place agranular effect may be produced by the complete 
interpentration of two individual crystals of the same size. In this case 
1 Mineralog. Magazine, vol. 5. p. 15. 
2 Journal of the College of Science, Imperial University, Tokvo, vol. 
I, part III, pp. 303-312. 
