168 GEOLOGY. 
the covering of loose earth, derived from their degradation. Slight exposures at other places 
show that nearly all the materials are soft and light, without much color, and similar to the 
strata described in the section. 
It was, however, observed that the upper beds of the formation, at other places, were gen- 
erally more argillaceous than.the lower. 
Plates of gypsum, from two to four inches thick, were found in the bed of the creek, mingled 
with the debris of the formation. This mineral was also found in place, forming thin layers 
parallel with the stratification, in several hills not far from the one from which the section 
just described was taken. These plates were generally crystalline and transparent, and were 
sometimes combined with the fibrous variety called satin-spar. 
From the examination ot this descriptive section, it will be seen that volcanic products enter 
largely into the composition of the strata. They even form the greater part of beds, ten to fifty 
feet in thickness. The fact is significant, and leads to a consideration of the mighty exhibitions 
of volcanic activity that must have been nearly co-existent with the formation of the strata. 
In some of the beds of fine, white pumice-sand, thin seams of charcoal were distinct. They 
were so numerous in some places that slabs of the sand were complely blackened by them. This 
charcoal was in mere fragments, generally so minute that their true character was easily 
overlooked ; but several pieces from one-quarter to one-half of an inch in diameter were found, 
and in some the cellular structure of the wood was easily seen. 
Stains produced by oxide of iron.—Layers, colored red by the presence of a considerable quan- 
tity of sesquioxide of iron, were abundant in different parts of the strata, and were often very 
thin and near together. Although these layers were generally conformable with the strata, 
and often marked out the divisions very clearly, I became convinced that they were generally 
the result of infiltration, rather than of original deposition. Of the correctness of this view, I 
considered that there was sufficient evidence in the curves presented by the layers of oxide on 
the vertical face of one of the thick beds of a light color, it thus being favorable for exhibiting 
the lines. After tracing these ferruginous layers for considerable distances in a horizontal line, 
some were found to bend suddenly downwards, in a curved direction, as shown in the annexed 
figure. They evidently followed the walls of a slight fissure, extending obliquely across the 
beds, and were undoubtedly formed by the gradual infiltration of ferruginous water, probably 
derived from the decomposition of pyrites. 
DEPOSITION OF OXIDE OF IRON BY PERCOLATION. 
Ld 
Fig. 1. 
The T fluid appears to have reached the fissure*in its descent, and then to cti 
flowed in greater quantity along its course, depositing the oxide of iron on the walls, and being 
