Prof. J. B. Harrison — Laterite in a Quartz-free Diabase. 355 



orange-yellow tinge in the inner parts, but change to brownish-red 

 and red translucent masses, with in places more or less opaque patches, 

 in the outer parts. The former areas of felspar are outlined along 

 the junctions of the former prisms of felspar by films of limonite, and 

 so are the former cleavages and the lines of chemical weakness. 

 The spaces between the films of limonite are completely filled with 

 glimmering masses or aggregates of very minute scales of gibbsite, 

 which mineral can also be detected more sparsely and irregularly 

 distributed between the ferruginous reticulations in the former 

 augitic areas. 



The higher powers of the microscope show in the part of the laterite 

 adjacent to the unaltered diabase a few minute fragments of unaltered, 

 not-stripped felspar, but these are of very sparing occurrence ; some 

 small areas of felspathic micropegmatite, in which one of the felspars 

 is more or less clouded whilst the other remains clear and transparent ; 

 small irregular areas or blebs of secondary quartz are of rather 

 frequent occurrence, especially in the immediate vicinity of patches of 

 micropegmatite ; occasionally, but rarely, quartz occurs in hexagonal 

 blebs ; a few long and very thin prisms of zoisite can be found, 

 whilst here and there are seen small patches of more or less chloritized 

 or bleached biotite. In this part of the lateritic crust the original 

 iron-ores are little, if at all, changed. By far the greater part of 

 the felspar is altered into aggregates of minute scales of gibbsite, the 

 nature of this mineral having been proved by comparison with gibbsite 

 in some slices of highly bauxitic laterite. Some of the augite masses 

 appear to be changed into or replaced by limonitic products en masse, 

 many of these strongly resembling goethite ; other portions are altered 

 into aggregates of talc stained with ferruginous matter, while in a few 

 places some of the augite has been altered to a light-brown mica; 

 a few fragments of unchanged augite remain. But the most usual 

 alteration of the augite is into interlacing streaks of limonitic hydrates 

 of iron with sparsely distributed patches of very minute scales of 

 gibbsite between them. The latter may be a product from small 

 crystals of labradorite included in the augite. In the middle and 

 outer parts of the laterite slices no remnants of unaltered felspar 

 or of micropegmatite can be found, while the prisms of zoisite are 

 indicated merely as ' ghosts ' ; the talc and the secondary mica in the 

 former augitic areas have disappeared and have been replaced by 

 limonite or other hydrate of iron, and no remnants of unaltered augite 

 remain. A few small patches of original mica still remain and are 

 but little altered. The titaniferous iron- ore has undergone some 

 changes, many of the granules showing evidence of having lost some 

 of their magnetite, only the ilmenite remaining quite unchanged. 

 The secondary iron-ores have in part segregated into aggregates which 

 are generally of a reddish-brown to a red colour and in places are 

 more or less opaque. Changes in the state of aggregation of the 

 gibbsite are also indicated, many of the scaly aggregates showing 

 rotating crosses under polarized light, and others show more marked 

 concretionary structure, while in a few places the hydrate of alumina 

 has assumed the form of angular platelets, some of which appear to be 

 roughly hexagonal. Small granules of quartz exactly similar to those 



