448 Professor J. B. Harrison—‘ Laterite’ in British Guiana. 
cut through them for the purpose of conveying water for the hydraulic 
workings. Their mass was quite soft when first dug into, but 
gradually became indurated on exposure to the atmosphere to the 
consistency of soft rock, so that the sides of the ditch and the sides 
and roofs of the tunnels became firm enough to allow of the rapid 
passage of water along and through them without any artificial 
supports being required either for the sides of the trench or the roofs 
of the tunnels. 
The hardening was accompanied by a gradual darkening in the 
colour of the rock, this indicating changes in the states of hydration of 
the oxides of iron present in them. 
At Omai these earths are largely the residua from the decomposition 
of sericitic and of chloritic schists, which latter frequently contain 
sericite in considerable proportions. 
At Omai the rocks are completely decomposed to depths of 100 to 
150 feet, and owing to this and to the intricate nature of the complex 
of sericitic, hornblendic, and chloritic schists, epidiorites and porphy- 
roids, traversed by veins of sericitized aplite and felspar-porphyrite 
and by sills of diabase, it is not possible to ascertain which rock or 
rocks by decomposition gave rise to the sericitic earths. Hence 
I have not analysed the specimens of sericitic and other schists of 
which the cores of the diamond-drill borings obtained at Omai largely 
consist, and therefore it is not feasible to contrast the compositions of 
the residual earths with those of the rocks from which they have 
been derived. 
The ultimate compositions of representative samples of the sericitic 
earths are as follows :— 
Taste VI. 
3 
Brownish Yellowish Yellowish . 
red. brown. brown. Purplish red. 
Oyen J oc. 6 6 ai 31°44 37°28 7°05 
Colloid Silica 30 “14 “23 “08 
Combined Silica . . 24°04 19°58 16°54 14°53 
Aluminium Oxide. . 23°94 27:21 24-98 21°34 
Iron Peroxide . . . 24°65 8:78 7:57 39°03 
Magnesium Oxide. . “30 23 nme “21 08 
Calcium Oxide. . . 10 06 06 05 
Sodium Oxide... . 10 trace | trace “25 
Potassium Oxide . . ODT 1-56 | 1°85 2°58 
Wiater’ ot Sees 11°70 7g) WORT 13°22 
Litanium Oxide . . 2°31 6d | ‘70 DOYS) 
Phosphoric Anhydride trace trace | nil nil 
Manganese Oxide. . nil nil nil trace 
| 
| 99°88 99°43 100-69 100°56 
The average proximate mineralogical composition of the earths when 
calculated out in the manner already described is as follows :— 
