ill the Urallan MouHtitins. 269 



>nust the rock be dolomite or not ? The arrangements Engel- 

 hardt observed in the year 1821, in the government of Olonez, 

 where black dolomite, containing drusy cavities lined with bit- 

 terspar and rock-crystal, resembling that of the valley of Adolph- 

 skoi, may enable us to try the first question. They occur on the 

 north-west side of the Lake of Onega, along with greenstone, 

 which is the predominating rock. 



In the Urals, Engelhardt, with exception of Kresdowos- 

 dwishenski, saw no black dolomite, probably it may be found 

 on careful examination in the brook of Suchoi Wissi, on the 

 west side of the mountains, where are situated the platina mines 

 of Nishnei-Tagil. Tiie quartzy chlorite-slate which occurs 

 there, much i-esembles theitacolumitein the vicinity of the place 

 where the diamonds are found, and both probably lie in the 

 same line of direction. If search was made there for diamonds, 

 although no dolomite or other carbonaceous rock was visible, it 

 would shew Avhether their appearance was always connected with 

 that rock formation or not. 



According to Gobel, the black dolomite from Valley of Adolph- 

 sJcoi contains 7.50 black powder, partly carbon, insoluble in 

 muriatic acid ; 40.79 carbonic acid ; 0.50 alumina; 6.28 oxide 

 of iron ; 30.65 Hme ; 13.05 magnesia ; 1.20 water ; = 99.97. 



Whether this rock, says Gobel, be viewed as of Neptunian 

 or Plutonian origin, the circumstance of its containing carbon 

 as one of its constituent parts, and the finding of diamonds 

 amongst its debris, is very remarkable. It may be asked if this 

 does not afford a hint as to the mode of occurrence, and the ori- 

 gin of the diamond ? The carbon is disseminated in an extremely 

 minutely divided state, so that we cannot determine the geome- 

 trical form of its individual particles, and which therefore can 

 only be derived from decomposed carbonic acid, but cannot by 

 any means be considered as remains of burnt organic bodies. 

 During the formation of black dolomite, very probably a great 

 quantity of carbonic acid was present, and it is not improbable 

 that a part of it on coming in contact with the bases of the 

 earths and alkalies, and with iron, would be deoxidized, whereby 

 carbon in substance would be separated, and would, along with 

 undecomposed carbonic acid, unite with these oxides. The de- 

 composition of carbonic acid by kaliuni and sodium has been long 



