“Crosby.] 492 [April 7, 
ezuela known as the Caratal Gold-field, with the district one hun- 
dred miles broad lying between it and the Orinoco. 
During the years 1867-73 a geological survey of British Guiana was 
carried on, under the authority of the British Government, by C. 
Barrington Brown and J. G. Sawkins, both men of large experience 
in the geology of the northern part of South America and the West 
Indies. The scientific results of their explorations, including a geo-— 
logical map of the Colony, were published in 1875. 
Although this report represents several years of hard work, yet 
the difficulties encountered were so great that we have here, so far 
as it relates to the geology, at least, little more than a sketch. All 
the impediments to exploration naturally pertaining to a wilderness 
under the equator had to be surmounted, including the almost impen- 
etrable tropical forests which made the rivers, with their numerous 
rapids and cataracts, with slight exceptions, the only practicable 
routes of travel, and including also the absence of even approxi- 
mately correct maps of the territory. | 
There is, however, a grand simplicity in the geology of this region 
which gives a mere outline sketch much greater value than it would 
otherwise possess; and, although so much remains to be done, yet the 
main geological features of the Colony appear to be pretty well made 
out. The line between the crystalline and the uncrystalline forma- 
tions is strongly marked and unmistakable, the contrast being even 
greater here than in Brazil. 
The coast is fringed at all points by a belt ten to seventy miles 
broad of Post-tertiary sands and clays, This follows the valleys of 
the principal streams long distances into the interior, and is the only 
fossiliferous formation in the Colony. Covering an area of about 
10,000 square miles in the western central portion of the Colony, 
and forming the main mass of the Pacaraima Mountains, is a great 
sandstone formation. “It is composed of interstratified beds of coarse 
conglomerate, red and white sandstone, and red shale. Interbedded 
with the sandstone are three great layers of “greenstone,” partly 
contemporaneous and partly intrusive. This greenstone is a coarse 
variety of diorite (or diabase), composed of hornblende (or augite) 
and feldspar. The total thickness of the sandstone formation, includ- 
ing the green-stone, is about 7,000 feet; the three greenstone layers 
varying in thickness, in different localities, from 400 to 1,000 feet each. 
The sandstone, as a whole, lies in a nearly horizontal position, dipping 
northwardly with but few local disturbances, and, so far as could be 
