110 - GEOGNOSY AND GEOLOGY. 
structure, but are only modifications of trachytic tufa. Thus we have 
peperino, which presents an ash-grey cement, whose uniformity is 
interrupted by slaggy particles and glassy feldspar ; also the pausilippo-tufa, 
of a yellow color, and frequently possessing a certain porosity ; together 
with the Rhenish tufa, tras or terras, in which pieces of pumice are joined 
together by a grey earthy mass. 
Basalt Conglomerate (trap tufa, Tuff-wacke). This reck is a conglutinate 
of basaltic fragments and of various other rocks, which are combined by an 
attrition-product of the basalt. Vesicular basalt is of most general occur- 
rence, as also clinkstone trachyte, granite, sandstone, mica slate, and quartz. 
Augite, basaltic-hornblende, olivine, wood-opal, brown coal, and some other 
mineral substances, are often distributed throughout the earthy mass. The 
hardness is inconsiderable. 
Leucitophyre Conglomerate. The character of this rock is much like 
that of the preceding. Fragments of leucitophyre, or of similar rocks, are 
combined by pulverized leucitophyre substances. Leucite frequently occurs 
in perfect crystals, as also mica and augite; more rarely melanite and 
hauyne. 
Pumice Conglomerate. This is composed of pumiceous matter cemented 
by a clayey substance. It is generally very light and porous, but, with some 
degree of hardness, furnishes a good building stone. 
b. Congregates. 
These are combinations of different particles which possess so little 
“oherence as to form soft, light, or loose aggregates. Congregates are 
neteronomic masses in which the cement is wanting. ; 
Series 1. Clays. 
The clay of the mineralogist occurs also in masses entitling it to the attention 
of the geologist. It is a silicate of alumima, with a varying proportion of 
water, contaminated by a number of different substances whenever it occurs 
in large quantity. Among these substances are especially to be found lime, 
coaly and bituminous particles, oxyde of iron and sand, which may be 
separated by washing. A preponderance of any particular ingredient 
determines its character, as: 
Iron Clay, which contains a considerable proportion of oxyde of iron, and 
is therefore of a reddish-brown color. 
Marl Clay, contaiming a large amount of carbonate of lime, and 
exhibiting various modifications, as the ferruginous, common, sandy, and 
bituminous. 
Drawing-Slate. This is a clay slate penetrated by a considerable 
quantity of carbonaceous matter. It produces a streak, and is brought 
into trade under the name of black chalk. It is of a pure black color. 
Bituminous Shale, or burning shale. This is a clay shale, impregnated 
by bituminous matter. It is distinguished from drawing-slate by its power 
of burning with a flame. The color is rather brown than black. 
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