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



It is evident from the above that the augite of the diabase is either 

 more calciferous than is shown by the calculation of the rnineralogical 

 composition of the rock from its chemical analysis given on p. 121 

 of this Magazine for March last, or else that I have not fully allowed 

 in my measurements for the proportion of labradorite included in 

 the ophitic areas of augite. The proportion of felspar found in the 

 various series of measurements ranged from 44 to 50'5 per cent., those 

 of augite from 44" 1 to 44-8 per cent., of biotite from -5 to l - 9 per 

 cent., of olivine from nil to l - per cent., whilst those of titaniferous 

 iron-ore ranged from 4"7 to 13*7 per cent. The interstitial quartz 

 detected varied from nil to - 08 per cent. 



The slices have been most carefully searched for signs of incipient 

 weathering. These are found to be more frequent and more marked 

 in those parts of the slices which have been cut from the outer part 

 of the diabase in contact witb the crust of laterite than in the inner 

 ones. The signs are far more marked in the felspar prisms than in 

 the masses of augite. The few granules of olivine also show well- 

 marked signs of incipient alteration along their cleavages. 



I was not able to trace satisfactorily the changes along the cleavages 

 and the lines of ' chemical weakness ' of the felspar by the use under 

 polarized light of ordinary microscopic objectives, althougb I used 

 various powers up to and including a one-twelfth inch homogeneous 

 immersion objective ; but finally I succeeded in getting good results 

 by the use of a Zeiss 4 mm. apochromatic objective with a No. 12 

 ocular, and possibly even more satisfactory ones using the same 

 objective with a "W. Watson and Sons' B analyser-eyepiece. 



Among the weathering products present are a few scattered minute 

 flakes of a chloritic nature ; while the lines of chemical weakness and 

 some of the cleavages in the plagioclase felspar are marked by very 

 minute colourless scales and lamellae of a mineral of somewhat higher 

 refringence and distinctly higher birefringence than is the labradorite. 

 These scales show a somewhat inclined extinction. 



From their general properties the minute scales appear to be gibbsite 

 or hydrargillite, a mineral which I have previously observed as a 

 secondary pi'oduct in some of the felspars of certain British Guiana 

 rocks. I cannot detect any signs of carbonates in the slices. The 

 weathering effects on the augite are more difficult to detect than are 

 those in the labradorite. But along the cracks and cleavages of some 

 of the augite masses there are minute scales similar in general character 

 to those in the plagioclase, but which are of lower refringence and of 

 somewhat lower birefringence than is the pyroxene ; the major part, 

 however, of the incipient weathering products in the augite are more 

 or less opaque and are difficult or not possible to differentiate. 



The Structure of the Laterite. — As already mentioned, the slices were 

 cut through both the inner and the outer crusts of laterite so that it is 

 possible to trace the changes and the differences in them. Under 

 a low magnification the slices show very clearly the structure of the 

 rock from which the laterite was formed, and this is far better 

 marked in its inner than in its outer part. The former masses 

 of augite are now occupied by reticulated intersecting lines and 

 streaks of limonitic iron-ores which are of a more or less transparent 



