Vol. 55.] INTO DIABASE AT SOREL POINT (nOETHERN JERSEY). 443 



of matrix to the other constituents, and corrodes the felspars. 

 Zircons are seen not uncommonly in the larger flakes of hornblende 

 surrounded by dark halos. In other sections, the younger rock 

 contains less hornblende and mica. 



The changes produced in these granite-veins are much less 

 marked than those found in the diabase. Quartz in large irregular 

 grains is present as in the normal rock, and bears the*same relation 

 to the felspar. This shows occasionally a microperthitic structure, 

 and less than might have been expected of the curious breaking- 

 up and reconstitution described before (p. 437). Plagioclase is 

 fairly abundant, and, judging from the low extinction-angles, is 

 oligoclase or a closely-allied form. Here and there little patches, 

 consisting of biotite and hornblende, are found identical in character 

 with the same constituents in the more basic side of the junction. 



The apparent facility with which important mineralogical changes 

 are induced in the diabase is rather remarkable. The basic rock 

 close to the junction is of an entirely different type from the diabase 

 in its normal state. The early stages of the change are illustrated 

 by a thin section cut from the heart of a clearly-defined wedge- 

 shaped fragment, some 3 inches long, lyiug with others like a 

 breccia in the pink granite. For instance, biotite, so common when 

 alteration has proceeded further, is scarce or almost entirely absent ; 

 the augite has been replaced by green hornblende, leaving ferru- 

 ginous dust and a slight suspicion of a fibrous structure as the sole 

 remnants of its former existence. The hornblende is, however, 

 often in a finely-granular condition, implying total reconstitution 

 of the original pyroxene. This variety has a tendency to become 

 idiomorphic. Occasionally small biotite-flakes are embedded in the 

 hornblende- substance, in a way that strongly suggests their secondary 

 nature. Plagioclase abounds, so that the only traces of the foreign 

 magma consist in the occasional presence of quartz-grains together 

 with some untwinned felspar. Hornblende is by no means absent 

 from these patches, which are closely dovetailed into the surround- 

 ing rock. Iron-oxide, in the form of grains and rectangles, is 

 scattered abundantly through the section, and the colourless 

 acicular crystals noticed before (p. 442) are not absent. 



Few new points are shown by another section cut from the same 

 specimen of a fragment in the later stages of dissolution, when it is 

 interesting to note the great preponderance of biotite over horn- 

 blende, the difference being apparent at once, even to the naked eye, 

 on placing the two slides side by side. The large quantity of mica 

 present when the acid element is in excess of the basic is a well- 

 marked feature in many rocks from the neighbourhood. Clusters 

 of hornblende-crystals and grains are found here and there, but not 

 abundantly, and may be taken to represent the remnants of diabase- 

 fragments. The presence in the hand-specimen of shreds, frag- 

 ments, and irregular patches of basic material renders such a 

 conclusion practically self-evident. A. few corroded plagioclases, 

 occasionally zoned, most probably are partially absorbed felspars 

 from the diabase. 



