Prof. J. B. Harrison — Laterite in a Quartz-free Diabase. 353 



IV. — Formation of a Laterite from a practically Quartz-free 



Diabase. 



By Professor J. B. Harrison, C.M.G., M.A., F.G.S., F.I.C. 



Part II. Microscopical Evidence. 



rPHE chemical evidence with relation to the formation of a laterite 

 J_ from a diabase was given in a paper in the March number of the 

 Geological Magazine, pp. 120-3. Since that account was written 

 I have made microscopical examinations of the materials therein 

 described. 



Thin sections of the diabase and of its crusts of laterite were 

 specially prepared for me by Messrs. Yoigt and Hochgesang of Gottingen, 

 the slices of the laterite being cut through both the inner buff-coloured 

 layer and the outer red one. 



Structure of the Diabase. — AVhen seen under the microscope the 

 diabase proves not to be as fine-grained as it appears to be in the 

 hand-specimen. It is a moderately fine-grained ophitic diabase or 

 dolerite, in which the pyroxene has some tendency towards an idio- 

 morphic or in parts a granular structure. The pyroxene is a colourless 

 augite of high double refringence ; in a few places its masses are 

 bordered by minute patches of a greenish biotite, some small flakes 

 and aggregates of which also occur here and there in the rock. The 

 felspar appears to be almost entirely labradorite, although there are 

 a few patches of non-striated felspar which may consist of orthoclase ; 

 these patches show some slight strain effects. There are a few small 

 interstitial patches of a micropegmatite of two felspars ; I could not 

 detect with any certainty any micropegmatite of felspar and quartz, 

 but there may be some present. There are also a few minute inter- 

 stitial patches of augite with some undifferentiated matter. Most of 

 the labradorite is in the form of relatively long prisms which vary 

 to a considerable extent in their breadths, some resembling in their 

 sections broadish plates. There are some large, very irregularly 

 shaped granules of titaniferous iron generally of ilmenite type, and 

 these are very irregularly and unevenly scattered through the rock. 



As an unimportant accessory constituent a few small granules of 

 olivine are present, whilst some long and very narrow prisms or 

 needles of zoisite, and exceedingly rarely a short prism of apatite 

 occur. Careful search has been made for quartz, but only a very few 

 minute interstitial grains of that mineral have been detected ; these, 

 however, appear to be of secondary origin. 



Numerous measurements have been made of the minerals present 

 in the slices, with the following results, given in percentages of the 

 constituents of the diabase by weight : — 



Felspar . 

 Augite . 

 Biotite . 

 Magnetite 

 Olivine . 

 Quartz . 



decade v. — VOL. VIII. — NO. VIII. 23 



