224 E. HILL AND T. G. BONNEY ON THE 



Under the microscope it is seen to consist of a rather finely crys- 

 talline ground-mass of somewhat altered small plagioclase crystals 

 interwoven with viridite, containing scattered magnetite, decomposed 

 ilmenite (?), and several fairly large crystalline grains, which, on 

 examination, are found to be made up of aggregated hornblende ; 

 this is feebly dichroic, and its general aspect suggests that it is a 

 secondary product. Except in this respect the rock still much re- 

 sembles some of the more compact less basic plagioclase basalts. 



Knoll of Diorite, north of Brazil Wood. — This rock is in gocd 

 preservation, and remarkably tough and refractory ; it is distinctly 

 crystalline, rather variable in texture, but generally coarse, dark in 

 colour, weathering brown ; macroscopically consisting of whitish 

 plagioclase felspar, with glittering faces, and black, fairly lustrous 

 hornblende. Under the microscope the latter mineral is seen to 

 occur in irregular grains, plate-like in section, pierced and intersected 

 by the plagioclase, with a little iron peroxide (?some of it ilmenite). 

 The hornblende with transmitted light is both brown and green in 

 colour, and much of it and of the felspar is still in good condition. 

 Here and there, however, patches of the pale fibrous actinolitic 

 variety occur, associated with a mineral rather like a fibrous variety 

 of serpentine. These last are certainly secondary products, and 

 much resemble a combination common in the altered Cornish 

 gabbros. Most of the larger hornblende is strongly dichroic ; some 

 of the brown variety resembles biotite. One or two pieces, indeed, 

 appear from their optical properties to be that mineral. There is a 

 little calcite with characteristic cleavage. The plagioclase often 

 shows brilliant colours and characteristic twinning, with occasional 

 indications of a zonal structure. Microlithic enclosures are frequent. 

 The microscopic examination can scarcely be said to confirm the 

 relationship between these two rocks, which is not only suggested 

 by their juxtaposition, but also favoured by the occurrence of diorite 

 boulders in the wood between the two localities. It is remarkable 

 that two rocks unique in the district, the above diorite and the 

 curious gneiss* already described, occur within a very short distance 



* The microsopic structure of the Brazil- Wood gneiss is described at p. 788. 

 Since the publication of that, Mr. Houghton has kindly favoured us with the 

 following analysis : — 



Composition of the Bock dried at 100° C. 



per cent. 



Water 4*23 



Si0 5401 



A1 2 3 21-87 



Fe 2 3 5-38 



FeO 624 



MnO 63 



CaO 2-13 



MgO 1-30 



K 2 36G 



Na 2 , 100 



Total 100-45 



The sp. gr is 2-8. p . 



