Professor J. B. Harrison—‘ Laterite’ in British Guiana. 561 
a 
in the form of hydrate in the earths of temperate countries the 
investigations by Liebrich of the occurrences of bauxitic material 
in the Vogelberg and Westenwald (Zeitschrift fir Krystallographie 
und Mineralogie, xxiii, p. 296, 1894, and Chemisches Centralblatt, 
1892, p. 94), where the bauxite was proved to be a decomposition- 
product of a basaltic rock, and the recent paper on the same subject 
by J. R. Kilroe (Gon. Mag., No. 534, p. 534, December, 1908), may 
be of interest. 
My long experience in the Tropics with igneous rocks and their 
decomposition-products has satisfied me that all questions relating 
to the degradation of the rocks and the re-arrangement of their 
decomposition-products may be accounted for by the normally 
occurring, practically unlimited factors of water, carbonic acid, 
decomposition-products, including organic acids of vegetable debris, 
and, above all, duration of time. Geologists and possibly chemists 
are apt to underrate the decomposing, ionizing, or mass action 
exerted by even pure water on rocks during very prolonged periods 
of time—periods which on the geologically very ancient land of the 
Guianas may have extended over geological ages.? 
There is no necessity for calling in the aid of the very small 
quantity of nitric acid supplied by the rainfall, which in British 
Guiana has amounted to only 9 pounds of nitric acid (H NO.) per 
acre per annum according to our monthly analyses of the rainfall 
carried on continuously for over twenty years. Nor are the small 
quantities of sulphuric and sulphurous acids derived from the 
oxidation of pyrites of importance, as in British Guiana pyrites is only 
present in appreciable quantity in metamorphosed rock, whilst in 
masses, dykes, and sills of diabase on which wide areas of laterite 
occur, its presence is practically confined to narrow selvages of the 
contact rock in even minute quantities. 
As far as the studies made in the Guianas go it appears that the 
cause of the hardening or setting of certain laterites will not be found 
by chemical analysis. My personal opinion is that it is due in part 
to changes in the degree of hydration of the hydrated oxides of iron 
' Some ten or eleven years ago, whilst lecturing on agricultural science, I used the 
following experiment to illustrate the action of pure water on rock. Rock powder 
was prepared from various types of rock by grinding on a bucking plate. About 
twenty grams of the rock powder was placed in a beaker, and from 100 to 150 ce. 
of cold, recently distilled water poured on it. To the water thus freed from carbonic 
acid a few drops of a solution of phenol-phthalin was added. In the course of 
a few minutes the water commenced to change to purple, and after ten minutes or 
so had elapsed the depth of colour produced served as a measure of the rates of 
decomposition of the various rocks by the water. I found that the most readily 
decomposable rocks were felspar-porphyry and porphyrite, the next being granitite- 
gneiss, followed by granite and granitite. The basic rocks were more slowly attacked 
by the water. After standing for some time when the colour of the water had 
ceased to deepen it was then poured or filtered off completely, the rock powder again 
treated with fresh quantities of the boiled water or in phenol-phthalin, when the 
gradual colourization again ensued. This, if desired, could be repeated many times, 
using the same rock powder. The experiments well illustrated the action of water 
free from carbonic acid on the rocks, and its ‘repetition with successive quantities of 
water show it to be a mass action. The rates of decomposition of the various rocks 
thus indicated were found to correspond with the extent of their denudation and 
degradation on the lower-lying lands of the colony. 
DECADE Y.—VOL. VII.—NO. XII. 36 
