108 ~  GEOGNOSY AND GEOLOGY. 
throw the cement into the background. The parts combined are often so 
minute as to permit the passage into iron sandstone. It is generally found 
in peculiar forms, particularly of tubular and stalactitic shapes. A remarkable 
variety is exhibited in the iron-stone conglomerate which is found in Brazil, 
and there termed tapanhoacanga (negro-head). It consists of pieces of 
specular iron, micaceous iron, and magnetic oxyde, cemented by red or 
brown iron-stone. Among the foreign admixtures of this rock are gold and 
the diamond. It is from this rock that the diamonds of Brazil and the East 
Indies are generally obtained. 
Granite Conglomerate (regenerated granite, arcose). Crumbled, weathered 
granite, the feldspar of which has been entirely decomposed,. is often 
combined in such a manner by argillaceous oxyde of iron, or hydrated 
oxyde, as to present an appearance not unlike real granite. The hardness 
of this conglomerate is less than that of granite, sometimes being exceed- 
ingly loose in its texture. 
Porphyry Conglomerate. Angular or rounded pieces of more or less 
decomposed eurite or clay porphyry, are connected by an earthy mass, 
which itself appears to have proceeded from the decomposition of porphyry. 
The cement sometimes so completely permeates the cemented, as to render 
a separation impossible. With the porphyry are frequently fragments of 
clay and silicious slate, granite, gneiss, mica slate, &c. The general color 
is brown, often with light spots, resulting from decomposed feldspar. A 
solid cellular variety, permeated by silex, affords an excellent material for 
millstones. 
Trap Conglomerate. Fragments of trap rocks, principally porphyritic 
trap and porphyroidal trap, are cemented by a mass which appears to have 
been produced by the attrition of the trap. This cement is often so similar 
to iron-clay as to be difficult of distinction. Pieces of eurite and clay 
porphyry, as also of granite, clay, and mica slate, are often intermingled in 
the conglomerate. ‘The predominant color is reddish-brown with a violet 
tinge. 
Iron-clay Conglomerate. Fragments, partly angular, partly rounded, of 
the most different simple or compound rocks, are combined by an iron-clay 
of an earthy weak fracture. The cemented parts are principally pieces of 
quartz, feldspar, clay and silicious slate, granite, gneiss, flagstone, and various 
porphyries. Their size varies from the largest lumps to the grain of the 
finest sandstone. 
Grauwacke. This is a conglomerate which undergoes the widest modifi. 
cations. Lumps and fragments of the most various kinds are combined by 
a clay slate cement. There generally occur in it quartz, silicious slate, clay 
slate, feldspar, mica, granite, various porphyries, and other compound rocks. 
Quartz seems, however, to predominate. The fragments sometimes occur in 
such proportion as completely to hide the cement. Grauwacke varies greatly 
with respect to the grain: while, on the one hand, the rock is composed of 
no inconsiderable pebbles or rolled fragments, on the other, these are so im- 
bedded in the cement as to lie entirely concealed. The cement even appears 
at times to be coarser than the parts cemented. A grey color predominates. 
538 
