Vol. 56.] THE SOUTH-EASTERN COAST OF JERSEY. 315 



crystallization of the felspar and an occasional tendency to penetrate 

 the hornblende-grains recall slightly the ophitic structure of a 

 diabase. In the same way the augite-cores, occasionally found in 

 the larger hornblende-individuals, suggest some relation to such a 

 rock. A very close resemblance to this hornblende-plagioclase rock 

 exists in the fine-grained black dykes which cut the hornblende- 

 gabbro of the eastern coast of Guernsey. 1 Without entering into 

 details at the present time, it may not be out of place to enumerate 

 a few points of resemblance between the rocks of the two islands. 



Omitting the gneiss of Guernsey, we find a dolerite or gabbro to 

 be the oldest rock in both areas. In the south-eastern corner of 

 Jersey, as described above, a dioritic rock is associated with the 

 diabase ; in Guernsey, dykes very similar to this cut the hornblende- 

 gabbro, which is itself subject to great mineral variations. The field- 

 evidence indicates that these dykes were derived from the same 

 source as the gabbro ; in Jersey the two rocks are closely connected, 

 but the one is not seen to cut the other. In both islands the next 

 intrusion was, when solid, a felspathic rock, containing little or no 

 quartz, hornblende, or mica. In Jersey this intrusion resulted in 

 the production of a new rock, by a process of mixture which is 

 characterized by the development of coarse actinolitic hornblende ; 

 in Guernsey, rocks occur with the same peculiar elongated horn- 

 blendes, produced apparently by a closely analogous sequence of 

 events. Putting aside the diorites of Northern Guernsey, an intrusion 

 of a granite-magma succeeded the rocks mentioned, as in the neigh- 

 bouring island. In both Jersey and Guernsey this intrusion 

 resulted in the softening and local melting of the rocks intruded 

 into. 



In seeking for an explanation, we may adopt the hypothesis that 

 the differentiation of the magma formed regions always poor in 

 ferromagnesian minerals which became progressively more acid ; and 

 we may suppose that between the body of basic material now repre- 

 sented by the diabase and those more acid regions a magma of a 

 dioritic composition was present, forming the fine-grained granular 

 hornblende-rock referred to above. This probably passed little by 

 little into the augite-bearing rock. 



III. The Western District. 

 (1) General Characters of the Acid Intrusion. 



The intrusive rock of St. Elizabeth's Castle is a fine-grained red 

 aplite, with a very few porphyritic felspars, about ^ inch long. In 

 a thin section a considerable quantity of quartz forming angular 

 grains may be seen. 



An approach to a granophyric structure occurs when three or four 

 quartz-grains embedded in felspar polarize together. The brownish 



1 See the paper by the Rev. E. Hill, with an Appendix on the Microscopic 

 Structure by Prof. T. G-. Bonney, Quart, Journ. Greol. Soc. vol. xl (1884) 

 p. 404. 



