982 G, ATTWOOD.ON SOUTH-AMERICAN GEOLOGY. 
45, A Contrisvrion to Sourn-Ammrican Guotocy. By GrorcE 
Artwoop, Esq., F.G.8., Assoc.Inst.C.E., Mem. Am. Inst. M.E. 
With an Avpenpix by the Rey. Prof. T. G. Bonnuy, M.A., 
F.R.S., Sec.G.8. (Read June 25, 1879.) 
[Prars XXXTIL] 
Tuer paper which I have the honour to present to the Society, en- 
titled «« A Contribution to South-American Geology,” refers to a tract 
of country about 150 miles in length, commencing from asmall port 
called Puerto de Tablas on the Orinoco river in the State of Guayana, 
Venezuela, and taking a south-easterly direction into the interior as 
far as the Caratal Gold district, as shown on the Map (Pl. XXXIII.). 
Mr. Carlos Seigert, a German surveyor who has spent many years 
in the country, has lent me his notes in relation to the line of country 
I have surveyed and described ; and his observations confirm the map 
I have compiled. The elevations marked on the section have been 
computed by the aneroid and mercurial barometers, and they 
were carefully checked by the boiling-point test made with Casella’s 
hypsometers, as shown in the following Table (p. 583). 
After landing at Puerto de Tablas and climbing up the sandbank, 
large exposures of weathered rock-masses are seen. It is only after 
a careful examination that the rock is found to be crystalline and of 
igneous origin. Parallel lines of alteration are so distinct and so 
regular on the outer surfaces that, until the weathered portions have 
been broken off and the unaltered rock exposed, it is not possible that 
a definite conclusion can be arrived at. The rock presents a highly 
crystalline appearance, and consists chiefiy of felspar, with some quartz 
-and a dark green mineral. It contains 64°83 per cent. silica, and 6-80 
per cent. iron =protoxide 8-34 per cent., or peroxide 8°84 per cent. 
The rock may be called a syenite (with microline) ; but until Prof. 
Bonney kindly examined my microscopic slides I did not feel certain 
on the point, especially as I found the amount of silica to be unu- 
sually large and more than is generally found in syenites. 
On the south bank of the Orinoco river the syenite extends some 
50 miles east of the port of Las Tablas, and about 90 miles west, as 
far as the city of Bolivar, near which place a granitoid rock appears, 
and further west a true granite. 
Having left the river, “the road” * takes a south-easterly direc- 
tion, and passes over about two miles of tableland, the rocks being 
syenitic and the samo as those found at Las Tablas. 
An open plain, called by the natives ‘“‘ sabanah,”’ has then to be 
crossed, and a change in the rocks occurs. ‘The rock is of a greyish- 
white colour, and consists of felspar, quartz, mica, and a little oxide 
of iron ; it contains 71°50 per cent. silica, and 2°75 per cent. iron= 
peroxide 3°57 per cent. 
* T call ‘‘the road” the course taken by myself from the river to the interior. 
