164 



THE GEOLOGIST. 



granular quartz of the mountains near Strehleu to the west of Breslau, and 

 which there underlies the quartz alternating with it and vast masses of white 

 talcose schist, and is traversed by numerous veins formerly worked for clear 

 quartz. ' . ' , 



The schist which we have called " metamorphic" presents various 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 of oxide 

 of iron. This schist constitutes generally speaking the elevated plains called 

 " chapades." It rapidly decomposes as we shall see further on, and on this 

 account the lower parts of the valley, into which the rivers proceeding from 

 the chapades flow, are of an undulating nature, while their higher slopes are 

 more or less precipitous. _ t . 



The passages from metamorphic schist to clay-slate and schists containing 

 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. 

 r\ T ear the limit of these two rocks concretions and bands of specular iron-ore 

 often occur. In the Serra de Caraca fragments of crystalline schist occur as a 

 conglomerate in the itacolumite. 



This metamorphic schist contains also limestone, schistose specular iron and 

 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 crumbling 

 readily to dust, the former undergoing a chemical as well as a physical change 

 in localities particularly rich in iron. The hydroxide 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 in 

 the \ alleys 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 shoidd go on to such an 

 extenl 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 

 act ion of 1 he wal er 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. 



li is in the producl 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 chrysoberyl. 



Experience has taught the miners that diamonds are to be found in three 

 distinct -regions called Servieo da Serra, Servico do campo, Servico do rio. 

 In the first the rock worked for diamonds is a product of decomposition named 



