Vol. 55.] INTO DIABASE AT SOKEL POINT (NORTHERN JERSEY). 447 



oxample, however, which has been described in the foregoing pages 

 the evidence afforded by microscopic study, no less than that obtained 

 in the field, shows the great alterations that have taken place in 

 the composition of the diabase, and of the granite intrusive in it. 

 Starting from the former in an unaltered state, we have traced the 

 gradual change })rodaced by the penetration of the acid magma, a 

 change at first confined to nests and patches of foreign minerals em- 

 bedded in diabase other«^ise scarcely altered. As tlie intruder gains 

 in strength so is the older rock gradually overcome, till at last we have 

 found neither diabase nor granite, but a compound of both which is 

 intermediate in character between them. We have seen the acid 

 magma in abnormal states through the assimilation of fragments 

 of the diabase in varying degrees, and we have traced the steps which 

 lead from those to the normal rock. 



The phenomena seem inexplicable as any form of difi'erentiation 

 in situ, but indicate the local melting down, partially or sometimes 

 completely, of one rock by another of very diff'erent chemical 

 composition. 



I wish, in conclusion, to express my deep indebtedness to Prof. 

 Bonney for invaluable advice and help, especially in that portion of 

 my work- to which the latter part of the Introduction refers. My 

 sincere thanks are also due to him for his kindness in looking through 

 the manuscript of this paper, and for permission to have slides 

 prepared from several of his specimens from the Channel Islands. 



EXPLANATION OF PLATES XXIX & XXX. 



Plate XXIX. 



Fig. 1. Brick-red granite from Sorel Point, forming the later intrusion of 

 the acid magma. On the left is a group of quartz-grains ; the central 

 clearer spaces are also quartz, three or four adjacent grains polarizing 

 together. The large felspar on the right is a plagioclase. X 50. 

 2. Ophitic diabase from La Plaine, The section shows two felspar- crystals 

 embedded in augite, much of which is changed into hornblende. Near 

 the centre two grains of magnetite may be seen in the latter. X 50. 



Plate XXX. 



Fig. 1. Section of modified diabase from Le Fosse Vicq, containing large ortho- 

 clases derived from the granite. Part of one of these can be seen at 

 the top of the figure. On the right are several glooieruli of hornblende- 

 grains surrounded by dark biotite-flakes. The majority of the dark 

 flakes scattered though the section are hornblende. A small nest of 

 quartz-grains spanned by a mica-flake can be seen halfway between the 

 centre and the lower edge of the figure. The clear outer edges of 

 the felspars are well seen in several instances. Sillimanifce, wliich is 

 present in the section, cannot be distinguished in the flgm-e. x8. 

 2. Part of the section described on p. 441. The figure shows hornblende, 

 biotite, felspar, and quartz. Towards the left and upper part of the 

 field may be seen a wedge-shaped crystal of ho]-nblende. The silli- 

 manite cannot be distinsfuished. X 16. 



