^°^* 55'] I^^O DIABASE AT SOREL POINT (NOETHEEN JEESEY). 441 



frequently penetrate the diabase, provide abundant material for 

 study. A specimen from La Houle, showing the contact between 

 the diabase and a thin vein of granite within a few feet of an 

 obviously intrusive mass, proves (i) that no distinctly defined 

 boundary can be drawn between the two rocks ; (ii) that shreds 

 and fragments of basic material are thickly scattered through the 

 substance of the vein. 



A thin section shows at a glance that the diabase has been very 

 greatly altered, or indeed reconstituted. The augites and replacing 

 brown hornblende, conspicuous in the normal rock, are now rei)re- 

 sented by a multitude of flakes of biotite and green hornblende, 

 small and irregularly scattered through the section (PI, XXX, fig. 2). 

 The pleochroism of the biotite is straw-colour for vibrations at right 

 angles to the basal plane, and a dark greenish-brown for vibrations 

 parallel to it. The absorption is considerable. This mica usually 

 occurs in rather square flakes, as a rule with jagged and irregular 

 terminations ; not infrequently, in little scattered groups of films and 

 clots. The latter habit is still more common in the hornblende ; 

 in one case the grains of this mineral are aggregated into a 

 glomerulus recalling that described by Prof. SoUas.^ 



All the glomeruli seen in sections of these Jersey rocks consist 

 of hornblende with small and rare grains of iron-oxide. Occasion- 

 ally the circumferences of the glomeruli consist of brown mica 

 (PI. XXX, fig. 1). The mica and hornblende are present in the 

 slide in approximately equal proportions, the biotite often associated 

 with the hornblende in a manner which suggests its formation from 

 that mineral. Occasionally the hornblende, of the usual green type, 

 takes on a rather elongated form. A very important point in 

 connexion with these altered diabases is the presence of much 

 quartz late in crystallization, so as to form a kind of matrix in 

 which the other minerals are set. The felspars in the great 

 majority of cases have a clearer, more translucent border, and 

 frequently show a slight zoning in that part. The outlines that 

 they present to the quartz are irregular. They contain abundant 

 inclusions of the other minerals scattered through them ; and this, 

 together with the granular habit of the hornblende and mica, gives 

 the section an ' untidy ' appearance which is rather characteristic. 

 The iron-oxide, probably haematite, commonly forms long rectangles, 

 often very even and regular. 



Another section of the junction between the altered diabase and 

 a granite-vein, itself containing hornblende and some biotite in so 

 great a quantity as to be scarcely recognizable as such, shows a rather 

 diflferent structure, noteworthy, since the quartz in the more basic 

 rock is a rare instead of a common constituent. It seems possible 

 that this may be connected with the more basic character of the 

 intruder. Hornblende, biotite, and magnetite are present in the 

 altered diabase as before, but the form that they assume is much more 

 granular. The hornblende especially is found in small grains, 



1 Trans. Eoy. Irish Acad. vol. xxx (1894) p. 494 & pi. xxvii, fig. 16. 



