92 



ME. E. 11. WOETH OX THE 



[vol. lxx^ 





it is typical of the ' dark igneous ' series and in part agglomeratie. 

 Where this dyke dies out in the aplite-quany on the western 



bank of the Eedaven (Q. Ill) 

 Fig. 1. — Sketch of adjacent faces it dips counter to the shales 

 ofljoulderfoHiidatJL.3£X.YI, at the same angle as the 

 S.JS. 15, showing penetration aplite, and it appears as the 

 of shale (white) by igneous former occupier of the fissure 

 rock (dark). of which the aplite is now 



the chief tenant. It is ob- 

 vious that these dykes must 

 be intrusive rocks. 



So far, the broader rela- 

 tions of : dark igneous ' rock 

 and shale have been described. 

 It remains to state that every 

 sill or dyke bakes and alters 

 the contact-shales, and takes 

 up inclusions of those rocks. 

 Every sill or dyke also pene- 

 trates the shales at and near 

 the contact. Excellent ex- 

 amples of this are to be found 

 at LXXVI, S.E. 15. Here 

 boulders occur, in which the 

 igneous rock is seen to pene- 

 a good instance is sketched, the igneous 

 1, a & h). 



^Length, of boulder = 2 feet 



trate irregularly the shale 



rock being defined by shading (see fig 



The Petrology of the ' Dark Igneous * Rocks. 



With the possible exception of the dolerite, which is rather 

 outside the series, the ' dark igneous " rocks present a constant 

 character, or if the}" vary, there are all gradations between the 

 extremes. Their base is a cryptocrystalline ground-mass, from 

 extremely fine to felsitic. In this base there may be either no 

 lath microlites of felspar, or many, so many as almost to exclude 

 the felsitic base ; considerable variation in this direction may take 

 place in one slide, extreme variation may occur in one exposure. 

 Here it may be well to refer to a specimen from LXXYIII, 

 N.W. 2, long. 4° 4' 5J", lat. o0° 11' IV \ on Sourton Tors 

 (McMahon's locality 3). This is a grey mottled rock, with no 

 visible porphyritic crystals. The section shows a microfelsitic 

 base, with much granular sphene and a considerable quantity 

 of chlorite ; the mottling is largely due to magnetite, which is 

 irregularly distributed. In places a few small felspar-laths appear, 

 and these increase in number locally, until patches of the section 

 are almost wholly occupied by the laths. Felsitic and trachytie 

 texture thus reach characteristic development in the same rock, 

 and within the area of a single section. In both the felsitic 

 and trachytie bases there occur felspar-phenocrysts varying from 



