96 ME. R. H. WOBTH OX THE [vol. 1XXV, 



points of greatest pressure the black pigment has been more 

 freely developed (PL VI, fig. 10). It should have been noted 

 that igneous inclusions frequently show invading tongues and 

 bays of the ground-mass. 



Contacts of ' Dark Igneous' Rocks and Sedimentaries. 



Very little need be written as to these. The tendency is towards 

 the formation of quartz-mosaic and either pale or dark mica. At 

 LXXY1, S.E. 59 a, a vein from the ' dark igneous' rock, itself devoid 

 •of brown mica, and hence much paler than the type, meets a 

 radiolarian chert. At the junction is an area of varying breadth, 

 which at first appears as though it might belong to either rock, 

 but is really the most highly-altered part of the chert. There 

 is not much pigment in this. Radiolarian casts are frequent, 

 and in some instances the original structure has been preserved. 

 Beyond this the chert, for a width of 4 mm., is strongly banded in 

 brown, the pigment being minute mica, set in a finely crypto- 

 crystalline ground, the intervening spaces being a coarser quartz- 

 mosaic. The bands, which are ' cobwebby,' are bent and irregular, 

 not always even approximately parallel, but bending one towards the 

 other and meeting so as to enclose lenticular spaces. Within this 

 4 mm. a little black pigment also occurs : this becomes abruptly 

 more prominent, and largely displaces the mica in the rest of the 

 slide. Kadiolaria occur throughout, and frequently form ' eyes,' 

 around which the dark bands bend (PI. VI, fig. 8). 



Dolerites. 



The dyke LXXVL S.E. 82-80-^66-67-84, consists for the 

 greater part of a dark purple-brown rock ; but in places the red- 

 brown mica which stains it is practically absent, and there the 

 rock is pale grey. It varies much in texture, being coarsest 

 at 80, where it consists of rather stout laths of felspar (appa- 

 rently oligoclase), with some broader forms and irregular areas 

 of the same mineral. The larger felspars have been invaded 

 by red-brown mica, of which much is present in the ground-mass 

 .also. The other minerals are augite and green hornblende after 

 augite, apatite, sphene, and ilmenite. 84 may be taken as the 

 other extreme in grain: the felspar-laths are very small, the red- 

 brown mica very minute and uniformly distributed, there is a 

 fair amount of sphene, some grains of iron-ore are present, but 

 augite, hornblende, and apatite are absent. One specimen has 

 been found in which the rock is amygdaloidal, the amygdaloids 

 consisting chiefly of green hornblende, but with a little quartz 

 also. LXXVI, S.E. 58 shows affinity with the line-grained rock. 

 Microscopically the original structure is much obscured, but lath- 

 shaped felspars of fair size evidently lay with their axes in all 

 directions, and some stouter forms are seen. There is no evidence 

 as to what the original interstitial minerals may have been. 



