CHAPTER III. 
CLASTIC ROCKS. 
INTRODUCTION AND CLASSIFICATION. 
In this chapter it is proposed to consider all rocks other than igneous 
which occur in the region covered by this report. Volcanic tuff, sand- 
stones, limestones, and schists will he included in the description. All 
the material will be studied as varieties of rock, and from a lithologic 
point of view, without special regard to age or stratigraphic position. 
The pyroclastic rocks are treated of here rather than with the igneous 
rocks proper, because they are for the most part waterlaid, and are 
closely related to the sandstones and conglomerates of true sedimen- 
tary types. 
In the scheme of classification which follows, the rocks have been 
divided into groups according to their composition as revealed in the 
hand specimen and by microscopic examination, precisely as if one were 
dealing with igneous rocks. The lines of demarcation have neces- 
sarily been somewhat arbitrarily drawn, for it was found that the tuff- 
sandstone-limestone series is complete and that all members of the 
group are united by transition forms. The table on page 19 will show 
the general divisions, while further details will be given in connection 
with each group as it is taken up for discussion. 
The phyllites, schists, and manganif erous iron slates have been stud- 
ied only in general, but are believed to belong to the same lithologic 
series and are accordingly assigned a place here. 
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