164 
THE GEOLOGIST. 
(rranolar quartz Of the mountains near Strehleu to the west of Breslau and 
thich there underHes the quartz alternatmg with it and vast masses of white 
talcose schist, and is traversed by numerous vems formerly worked for clear 
^The'schist which we have caUed " metamorphic" presents va,rious characters ; 
first of all it contains quartz associated sometimes with chlorite, or with talc ; 
sometimes with mica : in the latter case it passes into mica-schist It is not 
necessary to point out amphibole, hornblende, and schorl ? among the varieties 
of this schist worked in searching for diamonds. 
Sometimes it loses its schistlike nature, and then contains a quantity ot oxide 
of u-on. This schist constitutes generally speaking the elevated plams called 
" chapades." It rapidly decomposes as we shall see further on, and on this 
account the lower parts of the vaUey, into which the rivers proceeding from 
the chapades flow, are of an undulating nature, while theii" higher slopes are 
more or less precipitous. . 
The passages from metamorphic schist to clay-slate and schists contammg 
talc, mica and cyanite are very frequent, and quite insensible. So on the 
other hand is the passage from the metamorphic schist to the itacolumite. 
Kear the limit of these two rocks concretions and bands of specular iron-ore 
often occur. In the Serra de Cara^a fragments of crystalline schist occur as a 
conglomerate in the itacolumite. , 
This metamorphic schist contains also limestone, schistose specular iron ana 
itabirite, which is simply a variety of specular iron, accompanied by quartz and 
mica. It occurs in thick beds of great extent, which can be worked as iron- 
stone. When this itabirite is found partially decomposed (pulverulente), it 
goes under the name of " jacotinga." The valuable English mine of Gongosocco 
is worked in this jacotinga. 
The limestone is well developed, and contains many caverns in which bones 
and saltpetre occurs. These have been studied by Dr. Lund._ 
The itacolumite and metamorphic schists occur generally in alternate beds. 
Their outcrop is parallel to the mountain-range, namely north and south, and 
they dip to the east. They have been disturbed in such a manner that they 
form rugged and abrupt rocks which in the itacolumite are traversed by a great 
many water-worn channels. 
The schist and itacolumite decompose easily, the latter splitting and crumbhng 
readily to dust, the former undergoing a chemical as weU as a physical change 
in localities particularly rich in iron. The hydi'oxide of that metal when liberated 
goes to form, with the less decomposed fragments, a breccia, called ''lapanhoa- 
canga," a variety of which containing a greater quantity of sand is found m 
the valleys and rivers of the diamond region. 
This decomposition sometimes reaches a great depth ; the surface of the rock 
in the rainy season resembling a bog in which one sinks up to the knees. 
It may be thought surprising that decomposition should go on to such an 
extent in the tropics, as it is not seconded by the action of frost, but it is 
doubtless hastened by the frequency of the tropical rains, and the dissolving 
action of the water increased by the temperature. It may also be observed that 
the tropical storms of frequent and regular occurrence charges the atmosphere 
with nitric acid, which is communicated to the water and increases its action. 
It is in the product of the decomposition of these rocks, and in the recent 
lapanhoacanga that the diamond and numerous rare minerals occur. Among 
the latter we may mention topaz, chrysolite, amethyst, tourmaline, blue and 
black ore of titanium, transparent andalusite, euclase, and chrysoberj^l. 
Experience has taught the miners that diamonds are to be found in three 
distinct-regions called Seiwigo da Serra, Servipo do campo, Servi90 do rio. 
In the lirst the rock worked for diamonds is a product of decomposition named 
