586 G. ATTWOOD ON SOUTH-AMERICAN GEOLOGY. 
rocks, highly ferruginous and much altered. The extent of the same 
is not over a quarter of a mile in width, and then a rock is found 
with a green and dullish black crystalline mineral, and carrying a 
small quantity of iron sulphides. It contains 51:15 per cent. silica. 
The specimens which I was able to collect are considerably wea- 
thered, but the characteristic features are those of gabbro. It con- 
sists of saussurite and diallage, and is sometimes coarse-grained, 
fine-grained, or compact. When polished it has both a slaty and a 
spotted appearance. Prof. Bonney states that the specimen I sent 
him for examination “ has once been a gabbro.” 
Within one mile of the Potosi Company’s office the gabbro gives 
way to a greenish-coloured rock, very compact and fine-grained, 
consisting of felspar, augite, and chlorite, and containing small 
quantities of magnetic iron, also traces of iron and copper sulphides. 
The rock has been called a diabase. 
To show the effect of weathering and the chemical difference 
produced in the composition of this rock, I have had three com- 
plete analyses made from one stone; and as the three different por- 
tions analyzed show distinct lines of alteration, and as they are all 
from one compact stone, the results have proved of interest. 
Three analyses made from one piece of diabase rock found at the Po- 
tosi Company’s Mines (Spanish Guayana, Venezuela, S. A.), to 
show the difference in chemical composition between the portions 
weathered and the portion unaltered. 
Highly 
Unaltered. Weathered. Weathered. 
SITIO Wi ON Hee te URES Sa ene 49°57 41°77 43°46 
PANNE ca scee tn shoi cs Nags amesteuite 1a 15:37 19°34 18°39 
Peroxide of iron ,......... ee 13°21 20°43 
Brotoxide Of MOM ioe pag. + 12°34 4-63 
Protoxide of manganese.... traces. traces. traces, 
WopPelar tired cies ncwsie oosnels cee traces. traces. traces. 
JErhaaYey meen nie cis Seer eee 9°65 4-98 2:37 
IMI OMeSIAN We, pot astute SuseRccsaps TAL 501 3°46 
OLAS Mie mas alee ube toes 0-85 0-69 0°59 
SOs Ea aS aN cnn ant rene 1:99 0°33 0-14 
PUNT ee tlaee tbe eh ct dad le trace. trace. trace. 
WHTORING prea. sy vtacnweuens elders trace. trace, trace. 
Phosphoric acid fies Gin none. none. none. 
Water (combined) ........ 3°10 7°30 7:95 
Water (hygroscopic) ...... 0°17 2°55 3°39 
100-45 100-31 100-18 
N.B. The above analyses were made at the Metallurgical Labo- 
ratory at the Royal School of Mines, Jermyn Street. 
The loss of silica resulting from weathering compared with the sta- 
bility of alumina shows that silica is much more liable to atmospheric 
action than alumina. 
