STATE GEOLOGIST. Td 
softened shale extends to some considerable depth, say 8 to 10 feet, the 
best policy will invariably be to work only the softened portion of shale 
along the surface, neglecting the hard main body of the shale entirely. 
Such a shale has usually a greenish or reddish color and is so soft that it 
can be readily cut with a knife, like soapstone. A material of this kind is 
to be greatly preferred to an alluvial deposit of clay liable to all the caprices 
of sedimentation by flowing water. 
The color of unweathered ferruginous shale varies from dark gray 
and bluish gray, while the softened weathered shale assumes reddish and 
greenish shades. 
Fig. 5. Section showing the graduation of the hard shale in the in- 
terior of a hill to the soft layers on the surface. 
On examining a hill of such shale (see figure 5) we find, first 
nearest to the surface a red clay containing fragments of shale followed 
by softened shale. The hard shale contains the iron usually as the 
ferrous carbonate and at the same time it may contain concretions, 
often} to the size of a man’s head, of ferrous carbonate, which ‘of 
course are rejected. These “kidneys” of iron carbonate, if present in 
large quantities and in smaller lumps so that they cannot be removed | 
by hand or machinery, cause the shale to become worthless for all 
manufacturing purposes. Another impurity liable to be found is iron 
sulphide (pyrites) in the hard shale which in the oxidized, softened shale 
becomes iron sulphate. 
The ferrous carbonate of the hard shale becomes oxidized in the 
softened shale to the ferric oxide or hydroxide which gives to the shale 
its reddish appearance. Comparatively few shales contain iron solely as 
ferric oxide, one of the exceptions being the Bedford shale, a material 
much valued in the manufacture of front brick owing to the magnificent 
red color it assumes on burning. 
As to: the influence of the high content of iron in these shales on 
the cement to be produced, it may be said that as long as the content of 
iron does not run too high, say more than 10 per cent. of ferric oxide, the 
shales are to be preferred to clays lower in iron but higher in alumina, 
inasmuch as they cause the burning to take place at a much lower tem- 
perature and reduce the danger of spoiling the mixture in the burning 
by “dusting.” The only possible objection which can be brought against 
a cetnent made from ferruginous shale is the darker color, which, of 
course, is due to the high iron content. | 
