Vol. 56.] THE SOUTH-EASTERN COAST OF JERSEY. 309 



In places the hornblendes which the more acid rock contains are 

 rounded and granular, at others the mineral occurs in foliated flakes 

 producing a gneissose appearance ; while close at hand these may 

 be replaced, sometimes in a patchy way, by another type of a 

 long prismatic habit. These changes are, no doubt, due entirely 

 to local conditions which arise in near proximity the one to the 

 other. Thin sections show that the quartz present varies in 

 quantity, but on the whole it is not common. 



The felspars present some interesting features. With ordinary 

 light the plagioclases which constitute the dominant felspar fre- 

 quently exhibit in certain parts a slight difference in translucency 

 and a distinct position of extinction between crossed nicols. The 

 differentiated spaces form indented bays or isolated lakes in the 

 felspar-crystal, which then has partly idiomorphic outlines turned 

 towards them (fig. 1). These outlines correspond (as may be seen 



Fig. 1. — Plagioclase-crystal showing corroded spaces, and almost 

 entirely surrounded by flakes of hornblende and mica, three or 

 four of which are embedded in it ( x 38). 



best between crossed nicols) to part of the edge of a granule into 

 an aggregate of which the original crystal is in process of being 

 resolved. That this change has been facilitated in many cases by 

 straining and fracture of the crystals prior to consolidation appears 

 from the bending and disjointing which the plagioclase-twins 

 frequently show. The extreme stage in the process of corrosion 

 is reached when the substance penetrating the felspar, increasing 

 in strength, wedges the latter apart aud forms a felspathic base in 



