Marker : Lamprophyre Dykes in Westmorland. 267 



of thin slices shows several interesting features. There is 

 rather more mica than in the former dyke, and the felspar is 

 oligoclase instead of labradorite, while pseudomorphs after 

 olivine are uncommon. The rock encloses abundant grains of 

 quartz, always showing signs of corrosion. These might 

 conceivably be derived from the breaking up of the lumps of 

 quartz-felsite which are seen in the dyke ; but this is not a 

 probable explanation, for in no case is any of the felsitic 

 ground-mass seen adhering to the quartz. Corroded quartz- 

 grains are indeed common in the lamprophyre dykes throughout 

 the district, even where there is no quartz-porphyry, either 

 as enclosed patches or as separate intrusions ; and we must 

 infer that they became involved in the lamprophyre magma 

 while this was still in the subterranean reservoir from which it 

 has been drawn.* 



A more unusual peculiarity of this lamprophyre dyke is 

 the occurrence of very numerous little spherical structures, 

 about .vL. inch in diameter, which have evidently been steam- 

 vesicles, but are now filled in various ways. Some of. tjiem 

 are occupied by the ordinary secondary products of the rock — 

 calcite or quartz, or both minerals within the same cavity. 

 Many of the vesicles, however, are filled with a fine-textured 

 felspathic aggregate, which has certainly crystallized from 

 fusion. There is usually a border of biotite-flakes arranged 

 tangentially about the vesicle, and occasionally a flake projects 

 into the interior, or a felspar crystal may be found in a like 

 position. But the material within the vesicle proper is wholly 

 felspathic, and is of much finer texture than the body of the 

 rock, showing either a ' felsitic ' texture or a confused aggregate 

 of delicate divergent fibres. The interpretation of this is 

 clear. At a late stage of crystallation, when almost the whole 

 of the rock had solidified, the residual magma, which was 

 wholly felspathic in composition, broke its way into the 

 vesicles, displacing the steam, and there consolidated. In 

 some places it has not entirely filled the cavity, and there re- 

 mained a crescentic space subsequently occupied by calcite 

 or quartz of secondary origin. 



This oozing-in of the final residual magma into steam- 

 cavities is very common in Tertiary andesitic dykes in the West 

 of Scotland and elsewhere, and was first noticed by Dr. Teall 

 in the Tynemouth dykef , but it is not, so far as I am aware, 

 a usual feature in our lamprophyre dykes. The process which 

 it illustrates is of interest in relation to the origin of various 

 igneous rocks by differentiation. If the residual juice had been 

 by any means squeezed out and injected as a separate intrusion 

 it would have given rise to a rock very different from the lam- 

 prophyre with which it was so intimately connected. 



* Geol. Mag. 1892, pp. 485-488. 

 t Geol. Mag. 1889, pp. 481-483, with plate XIV. 



iQi2 Sept. I. 



