Nuneaton, Warwickshire. 155 



pencils out the cleavages in basal and prismatic sections ; or it occurs 

 as coarse irregular ropes traversing the crystal plates in directions 

 roughly parallel to the cleavages ; or the original hornblende has been 

 reduced to a meagre skeletal framework embedded in serpentine. 



As the opaque brown masses, hornblendic in colour, were suspected 

 to be, in part at least, limonitic, the cover glass of a section was 

 removed, the section cleansed of balsam, and a drop of strong 

 hydrochloric acid applied to the brown mass. In a very few seconds 

 the drop had become coloured a distinct yellow, as shown on its 

 absorption by white blotting-paper, thus proving the presence of iron 

 in some very soluble form in the brown mass, which was therefore 

 composed partly of limonitic matter. 



The abundant development of calcite plates in close association 

 with serpentine and limonite suggests its derivation from hornblende 

 rich in lime as well as magnesia, and therefore of the ' basaltic ' 

 variety. Relics retaining parallel cleavage traces in single system 

 (i.e. sections more or less nearly in the prism zone) give nearly 

 straight extinction with reference to the cleavage traces. (Pleo- 

 chroism: a = straw or pale yellow ; ji or 7 = light brown or dark 

 brown.) Narrow strips of a transparent substance, with recognizable 

 •cleavage traces, very pale yellow in ordinary transmitted light, and 

 showing intense brown absorption in plane polarized light, appear 

 to be unaltered portions of original hornblende which has suffered 

 bleaching. 



The decomposition of the hornblende appears to have proceeded in 

 two different ways — either (a) the whole mineral has altered by 

 hydration to serpentine ; this is the prevalent method ; or (b) the 

 magnesia alone has separated from the hornblendic molecule to form 

 serpentine; the lime has formed calcite (as plates and granules) or 

 epidote (as granules), the residual iron as the hydrated oxide 

 retaining the form of hornblende. 



In support of the latter suggestion it may be stated that the 

 serpentine contiguous with opaque hornblende relics is usually only 

 very pale green owing to its low iron content, whereas the colour 

 of the serpentine which has originated by method («) is always 

 a pronounced green or yellow-green. 



The proportion of serpentine is fairly constant, even when definite 

 indications of original olivine are absent ; moreover, it shows 

 a characteristic fibrous or 'bladed' structure under crossed nicols ; 

 the major portion, at any rate, of the serpentine may be referred 

 with confidence to hornblende. 



The murky-grey colour and complete opacity of much of the 

 limonite suggest admixture with leucoxene representing the titanium 

 of the original hornblende molecule. Some leucoxene is certainly 

 derived from the alteration of titaniferous magnetite. 



Rounded or elliptical patches of serpentine showing the 'mesh' 

 structure under crossed nicols occur embedded in the hornblende 

 relics, or very rarely in the felspar; they appear to represent original 

 •olivine, very subordinate in quantity. Olivine is probably also 

 represented in the serpentine of the base. The quantity of olivine 

 present is somewhat variable and always small. 



