*94 MR. R. H. WORTH OX THE [vol. lxXV, 



LXXYI, S.E. 7. Brown tourmaline is developed at junctions 

 between ' dark igneous ' rocks and aplite : it is confined to the actual 

 •contact and a very narrow width within this ; ciystals frequently 

 pass across the junction, lying partly in both rocks, such ciystals 

 being golden brown as regards the part within the ' dark igneous ' 

 rock, while the part within the aplite is blue (Q. Ill, 76). 



Chlorite is very rare, but has been found in LXXVI, S.E. 7G, 

 South Down. 



Anthophyllite is local, freely developed where it does occur, 

 a,nd certainly a secondary mineral. It occurs as groups of diver- 

 gent microlites, many having the appearance of sheaves of reed 

 very tightly tied about the middles ; a typical group from LXXVI, 

 S.E. 15, is figured (PL VII, fig. 17). Some groups consist of 

 ■comparatively few microlites. This mineral is found in the ex- 

 posures 71-75, 39, and on Sourton Tors. 



Flow-Structure in the ' Dark Igneous' Rocks. 



The weathered surface of some exposures shows banding parallel 

 to the strike (for instance, the offset 15-78 and the dyke or sill 39). 

 This banding is sometimes so close as to amount to little more 

 than a striation. When there are felspars of size sufficient to be 

 readily recognized, they are seen to lie with their longer axes 

 parallel to the bands. Microscopically these conditions are matched 

 in some of the finer- grained specimens, in which felspar-laths are 

 not freely developed. In these such microlites as are present tend 

 to segregate, and to form bands in which the individuals show 

 parallel elongation. Fig. 11 (PI. VI) is an example from LXXVI, 

 S.E. 15 c. Rhyolitic structure is by no means unusual in the sec- 

 tions : it is frequently emphasized lyv the differential development 

 of brown mica in adjacent flow-striae, as, for instance, in ' II.N.W. 

 Meldon, A ' from near 15, and also LXXVI, S.E. 76. Flow- 

 structure can also be detected on polished surfaces. 



Inclusions in the ' Dark Igneous ' Rocks. 



With the exception of 82-81 every dyke or sill is in part 

 •agglomeratic. Macroscopically there are two types. In the first, 

 which is well developed in parts of 71-75, weathered surfaces show 

 the inclusions very clearly, but freshly broken surfaces give practic- 

 ally no hint of their presence. In the second type, which prevails 

 at 56 among other localities, the igneous inclusions are grey and 

 clearly defined against the purple-black ground-mass, even on fresh 

 surfaces. In the first type, the mica which colours the rock is 

 freely developed, both in the ground-mass and in the inclusions ; in 

 the second type it is practically absent from the inclusions, but 

 pale-green mica may occur, the original character of the fragments 

 being the same in both. Fig. 9 (PI. VI) reproduces a jmotograph 

 of a polished surface, on a specimen from 56 b ( x 2\). The edges 

 of the inclusions will be seen to have been rounded : the larger 



