ui) 
Frazer. ] 258 [Jan. 4, 
droxidation of the pyrite by reaction with limestone in presence of air 
thus : 
‘(H,0); + O + (FeSQ,), + (CaCO,), = (CaSO,), + H,Fe,0, + (CO,).. 
In the region which I have studied, it seems to be as difficult to define 
any horizon or horizons of hydromica slate as to define an horizon of 
moisture, or of hard and soft rock. Hydromica slate in the counties before 
named occurs at any horizon, and in all possible relative positions to the 
limestone. So far as I have been able to judge the relations of these strata to 
the limonite, they are twofold: Ist, as a carrier of the pyrites, and perhaps 
other iron bearing minerals, which by their separation from the other con- 
stituents have been washed down and collected in the impervious strata 
actually beneath them. 2d, as the material out of which the impervious 
clays themselves have been produced. 
As to the age of these hydromicas, they appear to be represented in the 
rocks of several ages. There are some which seem to be intercalated with, 
and therefore of the same age as the limestone itself. 
Some are found geologically beneath the limestone and intercalated 
among the chlorites, quartzites, and orthofelsites of the South Mountain 
group. 
Crystallography in Sculpture. 
By PERSIFOR FRAZER, JR. 
(Read before the American Philosophical Society, January 4, 1878.) 
Mr. Spring an artist and a modeler in clay has conceived the ingenious 
idea of making the human head out of a number of plane surfaces of dif- 
ferent area, and he designs these models, iess to aid in teaching anatomy 
than to aid in teaching sculpture. 
There are some fifty planes more or less represented on the small models 
such as this here shown, and of course it is of value to be able to designate 
each of them by some succinct and comprehensive notation. 
It was suggested to Mr. Spring to apply to crystallography for this 
purpose. 
The head here represented is of a high Caucasian type, unless I am de- 
ceived, and the symmetry of the features is as striking as their representa- 
tion by planes is novel. 
Of course since a plane represents a certain portion of such variable 
parts as the flesh, lips, ear, &c., no crystallographic formula would repre- 
sent the same feature on each of two twins, nor would it represent the 
same individual in different frames of mind, and states of physical con- 
dition, viz.: angry ; sentimental ; after dinner ; after sleep, &c.; neverthe- 
less, a sort of rough approximation to his mean condition will enable if 
