Yol. 56.] THE SOUTH-EASTERN COAST OF JERSEY. 311 



supposition that differentiation of the magma resulted in the for- 

 mation of a region from which ferro-magnesian constituents were 

 almost entirely absent, but which contained those of felspars iden- 

 tical in composition with the species of the diabase or one slightly 

 more acid. Another region might be formed in which, when it had 

 crystallized, hornblende and mica would be absent, quartz present 

 in small quantities, and the felspars would be an acid plagioclase 

 and orthoclase. Movement in these parts of the magma would 

 result probably in the intrusion into the larger body of diabase of 

 either one of these varieties or of any mixture of them ; and hence 

 might arise the phenomena of corrosion and replacement seen in 

 some of the slides. On this hypothesis the corroded plagioclases 

 described in the foregoing section (p. 309) would belong for the 

 most part, not to the diabase, but to a less basic rock which had 

 been intruded into, and to some extent altered, by, one of still less 

 basic composition. 



At the same time some of the appearances of fracture and corro- 

 sion maybe due to movement during consolidation, and to variations 

 in temperature and pressure attendant on intrusion, which caused 

 part of the felspar-substance again to enter into solution. 



(3) Phenomena attending the Earlier Acid Intrusion. 



Having thus obtained some idea of the nature of the intruder, we 

 may pass on to consider a few facts connected with its passage 

 through the older rock. 



A thin section cut across a junction (rather sharply denned in the 

 specimen) shows a dark compact rock on one side, and a coarser, with 

 conspicuous actinolitic hornblendes, representing the intruder, on the 

 other. The passage from the former to the latter is more marked 

 between crossed nicols than in ordinary light. It is made apparent 

 by a sudden increase in the size of the felspars and of the hornblendes 

 (no augite at all is found in this section), and by the presence of 

 quartz wedged in between the other constituents. There is, 

 however, a little quartz in the more basic rock. Long plagioclases 

 in great number are found in the intruder, often embedded in a pale 

 brown-red felspar which appears to be orthoclase : sometimes quartz 

 takes the place of this orthoclase. Apatite is found in some quantity. 

 The hornblendes are fairly idiomorphic; the pleochroism is of a straw- 

 yellow colour with a slight tinge of green, to rather dark green with 

 a brownish tinge. The usual length is -\ inch, occasionally |- inch. 

 The hornblendes of the more basic part (the rock intruded 

 into) are granular, and accordingly seldom or never idiomorphic. In 

 the centre of one, a little rusty-red ferruginous matter surrounded 

 by an area of somewhat lighter colour represents probably the 

 lingering traces of augite. In other sections quite clear augite-cores 

 are found, but not commonly. 



Field-evidence shows that this is not an intrusion of the ordinary- 

 type. The younger rock, that with actinolitic hornblendes at about 

 4 inches from the junction, contains a band, not very clearly 



