430 MK. J. PARKINSON ON AN INTRUSION OF GRANITE [Aug. 1 899, 



29. On an Intrusion of Granite into Diabase at Sorel Point 

 (Northern Jersey). By John Parkinson, Esq., F.G.S. (Read 

 June 7th, 1899.) 



[Plates XXIX & XXX.] 



Contents. 



Page 



I. Introduction 430 



II. The Granite 432 



III. The Diabase 434 



lY. Eocks containing Derived Material 435 



V. Junction between the Granite and the Diabase 440 



VI. Comparison of the Phonomena with those observed in other 



Districts 444 



I. Inteoduction. 



Prominent among the many points of interest which the igneous rocks 

 of the Channel Islands offer for study are those that arise from the 

 intrusion of one rock into another. In Sark the relations of the aplite 

 to the pre-existing hornblendite are well known through the work of 

 Prof. Bonney & the Rev. Edwin Hill ^ ; in Guernsey the latter 

 Author"^ has touched briefly on the relations, apparently of rather 

 unusual character, existing between a granite and a dioritic rock ; in 

 Jersey M. Nouiy, in his work upon that islaud,^ has remarked upon 

 the complete penetration in more than one place of a ' diorite ' by a 

 granite. It is to one of these intrusions, that of the north coast, 

 that the present communication relates. 



In many respects both the rocks and the locality are favourable 

 for study. The contrast presented by the brick-red granite so poor 

 in ferromagnesian constituents as to be practically an aplite, and 

 the dark basic rock into which it intrudes, is sufficiently striking 

 to render the broad facts apparent almost at a glance. The magni- 

 ficent cliffs, under favourable circumstances of tide and weather, 

 afford ample opportunities for the study of these relations in greater 

 detail. Anyone, walking westward from St. John's, in the centre 

 and north ot the island, and taking the road to Les Meuriers, will 

 find, by turning to the right when reaching the sea and keeping 

 along the edge of the cliff", the relations of the two rocks displayed 

 as finely as he can desire. Looking down into one of the numerous 

 little bays which indent the coast, veins of red granite can be seen 

 piercing and shattering the dark rock. Sometimes they evidently 

 proceed from a larger mass of granite, which occupies for a short 

 distance the whole cliff-face ; more often the younger rock forms 

 the outlying shelving masses, seen at their best when left bare by 

 the falling tide ; but the constant alternation of the two rocks from 

 the edge of the sea to the top of the cliff is the most marked 



^ Quart. Journ. Geol. Soc. vol. xlviii (1892) p. 122. 



2 ihid. vol. xl (1884) p. 404. 



^ ' Geologie de Jersey,' 1886, p. 22. 



