446 Professor J. B. Harrison— Laterite’ in British Guiana. 
for a rock to be termed ‘ laterite ’—the fact that they are ‘‘ essentially 
characterized by the presence of free hydrate of alumina”; but the 
question remains: If it is allowable to term ‘laterite’ the earth and 
its pisolites which contain, in round figures, 24 per cent. of ‘ bauxite’, 
and the concretionary masses which contain 42 per cent. of it, what 
are their accompanying masses which contain only from 2 to 7 per 
cent. of bauxite to be called? Are we to find another name for these 
masses whilst they are in situ, or may we not reasonably include 
them with the other components of the residuary products of the rocks 
as a whole under a wide-meaning term ‘ laterite’ ? 
Another type of the deposits formed by the decomposition of 
diabase and of hornblende-schist in situ is illustrated by samples from 
Tumatumari, Potaro River; the Penal Settlement at Mazaruni, 
Mazaruni River; and the Omai Falls at the Omai Gold Mine, 
Essequibo River. Their compositions and those of the rocks from 
which they are derived are as follows :— 
TABLE LY. 
| Tumatumari. Omai Falls. Mazaruni. 
| Diabase. | Laterite.| Diabase. | Laterite. Hong Laterite. 
| | chist. 
| 
Quartz . « | 3320 47°36 6°50 38°66 7°60 32°51 
Colloid Silica | 06 19 “0: 
Combined Silica 47°99 3°30 46°75 | 7:90 44°10 14°93 
Aluminium Oxide 15°80 26°38 i7°16 | 18°41 15°94 84°14 
Iron Peroxide 3°08 10°67 4°27 22°35 3°84 
Iron Protoxide . . 11:20 8:26 8:26 T1056 
Magnesium Oxide . 5°63 All 6:10 |} 12 5°54 
Calcium Oxide . 9°58 93 7:46 | 11 9-60 
Sodium Oxide 2:09 14 2°50 | “5, 1°87 
Potassium Oxide . . . 0°60 “21 0°69 | 47 “08 
AWiaber ciated teaser o's sre |e, LO8S0 11°28 0-32 | 10-97 “30 
Mitannum Oxide. 5. . 0:40 67 0°32 +50 *30 
Phosphoric Anhydride . 0°008 | trace trace | trace ‘O1 
Manganese Oxide . . . | trace nil 0-12. | — nil trace 
| 
| | 
| 99-878 | 108-71 | 10045 | 99°81 99°74 100-32 
| 
The mineralogical components of the rocks and of their resultant 
laterites have been calculated as shown in Table V (p. 447). 
These residual earths are as characteristic of many of the residuary 
products of basic rocks as those at Issorora are of others. But in 
them, in place of the silica segregating out into masses of quartz, and 
only occurring to a very limited extent as fine gravel or sand, the 
quartz occurs in quantity as very fine angular gravel and sharply 
angular sand of very varying degrees of division, but mostly of 
exceeding fineness, dispersed through the mass, by far the greater part 
of the quartz being of secondary origin. In them, as a rule, the 
