CARBONIFEROUS FORMATIONS. 31 
result a brown dolomite or “short lime” which can not be distinguished 
from that formed by the dolomization of blue limestone. There is also a 
marked alteration observable throughout the whole zone of oxidation, which 
is apparently due to surface influences alone. In this zone the hard, firm, 
calcareous shale softens and loses some of its cohesion. In consequence cf 
this softening, the bluish-black color of the shale becomes dead black; this 
is accompanied by a partial alteration of the calcite in the rock and in the 
veins to gypsum. his change is well seen in many deep shafts which go 
below the zone of oxidation and penetrate the Weber rocks at some point 
where they are not softened in the vicinity of a watercourse. In the 
Smuggler and adjoinmg mines these transition steps were especially noted, 
the soft black ‘‘shale” of the upper levels becoming, at a depth of 700 or 
800 feet, a hard, black, argillaceous dolomite. The zone throughout which 
this softening extends is practically the same as that in which the oxidation 
of the Carboniferous and Silurian dolomites takes place and in which the 
alteration of the sulphides in the ores to sulphates, carbonates, and oxides 
begins. 
Microscopic structure— Under the microscope the Weber limestone is seen to 
be made up largely of cryptocrystalline carbonate, so fine in grain that no 
individuals can be distinguished, while some areas are more coarsely crys- 
talline. The manner in which these latter areas occur suggests regenera- 
tion, or crystallization by the same agents which have produced the veins 
of white calcite which are so profuse. Certain forms suggest organic origin; 
some of these are marked by the crystalline carbonate above noted, while 
others are distinguished by the presence of much opaque, dark, nearly sub- 
microscopic matter, which is probably carbonaceous and argillaceous. This 
carbonaceous matter is irregularly disseminated in the whole rock, to which 
it gives its black color. There are occasional small detrital grains of quartz 
and of zircon. ‘The quartz in some of the sections is rounded, while in others 
it has assumed the form of long, slender crystals. Since the origin of this 
material is apparently detrital, the crystals are probably formed by the 
building on of new silica to the original irregular grains. Pyrite is present 
in small crystals, sometimes distributed with apparent uniformity through 
the rock, but oftener concentrated along some weak and more porous zone. 
In sections where the organic forms are found the pyrite is often unmis- 
takably clustered in their vicinity. 
