1880. 493 [Crosby. 
seen, it is singularly free from faults and dislocations.” It is observed 
to rest with perfect unconformability upon the underlying crystalline 
formations, of the ruins of which it is composed. The sandstone 
forms the highest land in the Colony, including the remarkable ver- 
tical-walled table-mountain of Roraima. 
Roraima reaches an altitude of 7,500 feet, the last 2,000 feet be- 
ing perpendicular; and Mr. Brown claims that whatever animals may 
inhabit the summits of this and adjacent mountains of similar forma- 
tion must have been cut off from communication with the surround- 
ing country since a very early period, and therefore he finds it inter- 
esting to speculate upon the strange forms which the “ survival of 
the fittest” may have developed there. 
Concerning the age of this formation, Mr. Brown says, —“ The 
absence of fossils in the sandstone is very singular, and prevents one 
from arriving at the epoch of its deposition. It appears to me, how- 
ever, to be an equivalent of the new red (Triassic) sandstone.” This, 
it may be remarked, is the age which Mr. Derby assigns provision- 
ally to the precisely similar sandstone and trap formation of south- 
ern Brazil. Be 
With the exceptions named, all the rocks of British Guiana are 
erystallines; and these are arranged by Brown and Sawkins in three 
general groups which, beginning with the oldest, are designated as 
follows:—1. Granite and syenite; 3. Quartz—porphyry and felstone; 
3. Gneiss and schist. ‘The first group is one of great importance, 
covering about one-third of the Colony. It consists mainly of gran- 
ite, the syenite occupying a v@ry subordinate position. The granite 
varies greatly in texture, but it is usually coarse and often porphy- 
ritic; the feldspar is commonly white, but sometimes red; and, as a 
rule, the rock is little micaceous. The granite lies at the base of all 
the rocks of the Colony, and coarse veins of it have pierced all the 
overlying formations, including even the sandstone in one place. 
This rock is undoubtedly eruptive in the main, and yet, in the de- 
scriptions of Brown and Sawkins, there are plain indications that it 
is not wholly so. A gneissic structure and passages into distinct 
gneiss are mentioned as characterizing it at many points. What ap- 
pears to be the same formation has a large development in French 
Guiana, being probably continuous through Surinam, and it is here 
referred by Edouard Jannetaz to the same age as the oldest gneisses 
of Brazil, which are Laurentian. 
