Professor J. B. Harrison—‘ Laterite’ in British Guiana. 491 
TaBLE XII. 
| Mazaruni. Mahdia. Ropai | Ozealla. 
Upper | Couran- 
Mazaruni River. Potaro River. Mazaruni| lyne 
River. | River. 
: Pipe-clay| Pipe-clay 
Granite. |Pipe-clay Brom eae Pipe-clav| from u from 
; anite. A) eee : 
ranite. | Aplite. 
(Qyierti; aig | Gaede ae 36°6 38°7 29°5 38°1 48°3 6°7 
Colloid Silica 1°6 “4 14 3°8 
Orthoclase pecan MS 20°6 2-9 15°5 2-4 1:9 iL 
ae TOCaSe Ma yl a. Zo 374 37° 5 34 4:6 ~ 
INI CARMAN yet) os eats 13°3 3°3 
Hlornblende . . . . . 1207 
Nineties 5. Be ae Tea 
Iimoniteew we. °8 1°8 8 1°6 
Mitmentes =... 3 1°6 1:9 1:9 “02 
pSphenewes . sia) 4 SP 162 1-2 ; 
Meaolimitery Ys. 6 4s 50°2 48°6 42 80:4 
Tale Ab ears 3 Loy sil 2 
fmiauKigemeees Lh nil 4-9 nil 2 
Minor constituents. . . 7) 3 of) 
100°2 100 100 100 100°4 100°32 
ED IASMOLC Mee ses 14 
Gib bsiteyn fakes 3°5 2, 
{Total Alumina present in nil 4°6 nil oil 
Bauxite 
These products of granites decomposed in situ may be termed either 
decomposed granites, kaolins, or pipe-clays. They are white or 
creamy-white, soft, earthy rocks which do not harden on exposure to 
the atmosphere, but gradually crumble down when wetted by rain. 
They are mixtures of varying proportions of sharply angular quartz- 
sand and of clay-substance (kaolinite with a little finely divided 
felspathic rock powder, or flour), with usually small proportions of 
colloid silica, either without or with aluminium hydrate in more or less 
negligible proportions, with a little ilmenite. and with very small 
proportions of limonite. 
Another type of the residual products of the alteration, probably 
largely by deep-seated hydro-metamorphism of pegmatites and aplitic 
granites, is the sericitic one. This type is at times erroneously 
regarded as produced by ordinary processes of weathering. The 
rocks are glistening white, exceedingly soft, silky-feeling masses, 
which do not harden on exposure to the air, but quickly disintegrate 
to a sandy powder when moistened by rain or when placed in water. 
Their chemical compositions, their physical properties, and the 
