SEDIMENTARY FORMATIONS. 35 
eliminating from consideration the mica and iron oxide observed in 
the glass, it is easy to determine from the measured density of the 
rock that the density of the glass is not far above 2. Thus the 
theoretical complete crystallization of glass or fused equivalent in the 
contact phase of the limestone has involved a diminution in volume 
of perhaps 19 per cent, or about 15 per cent if pore space be taken 
into account. 
It may be concluded in general that the actual density of the 
altered rocks is less than that of the fresh limestone, largely because 
of the development of pore space and glass, but that the densities of 
the minerals of the altered rocks are on the whole slightly higher 
than those of the fresh limestone. 
Exceptional contact phases, consisting almost entirely of silica or 
garnet or other constituents, give other density results. 
If the principal chemical change in the development of the contact 
phase has consisted in the elimination of calcite and to a less extent 
of magnesia, iron, and potassa, leaving alumina and silica substan- 
tially unchanged in their ratios, as held to be possible in an earlier 
paragraph (p. 32), this has involved a very considerable loss of weight 
(see analyses, p. 27), and, as the densities of the fresh and altered 
rocks differ so little, the loss in volume also has been large. Kemp, a 
Lindgren, 6 and others have cited lack of structural evidence of dimin- 
nution in volume at limestone contacts as favoring the view that 
materials must have been introduced from without to take the place 
of the calcium carbonate. In the Iron Springs district the field evi- 
dence does not positively prove or disprove important volume change, 
but there is no apparent field evidence to contradict the evidence 
for diminution of volume here calculated. The limestone, though 
tilted away from the andesite laccoliths, nowhere shows evidence of 
crumpling or crowding where the bedding can be observed. In the 
altered phase the bedding has been destroyed, and it is easy to con- 
ceive that this structurally amorphous zone may represent only a 
part of the volume of the original rock, the calcium carbonate having 
been driven off and the other constituents concentrated. The 
change in volume of the limestone would scarcely be expected to 
stand out conspicuously in the field relations, for it has occurred, if at 
all, in' the band which now does not show original textures or struc- 
tures, by which change of volume can be measured in crumpling or 
folding. 
In general it appears that there may have been important dimi- 
nution of volume, accomplished essentially by loss of materials and 
not by change of density of minerals. 
a Kemp, J. F., Ore deposits at the contacts of intrusive rocks and limestone and their significance as 
regards the general formation of veins; Econ. Geol., vol. 2, 1907, pp. 1-13. 
b Lindgren, Waldemar, The copper deposits of theClifton-Morenci district, Arizona; Prof. Paper U.S. 
Geol. Survey No. 43, 1905, 375 pp. 
