part 2] GEOLOGY OF THE M.ELDON VALLEYS. 9& 



euhedral to formless. The interesting' feature of this rock is that 

 it supplies in one mass, and as variations of the same, two forms 

 frequent among the inclusions in the 'dark igneous' rocks. Further, 

 if the chlorite were replaced by red-brown mica, the rock would 

 be a typical non-agglomeratic example of the 'dark igneous' series 

 itself. 



In the 'dark igneous' rocks phenocrysts of felspar ma} T either 

 be entirely absent, as at chilled edges (for example, LXXVI, 

 S.E. 54, north of the viaduct, northern margin of the dyke or sill), 

 or may be present in numbers, either euhedral or anhedral : thus 

 LXXVI, S.E. 5-1 a, 2 feet within the margin of the dyke next to 

 54, shows many small felspar-phenocrysts, varying from euhedral to 

 irregular. As a rule, felspar-crystals 2 mm. long are exceptional, 

 but at LXXVI, S.E. 25, some slides show felspars measuring as 

 much as 10 mm. in length. At this place many of the felspars are 

 rather closely twinned on the albite plan, and appear to have been 

 considerably strained, the lamellae being thrown into gentle Avaves. 

 Bent and broken felspars are to be found in slides from practically 

 every exposure. Everywhere the felspars, when twinned, are mainly 

 in irregular and impersistent perieline lamelke : they are apparently 

 orthoclase, but probably, as MacMahon believed, with intergrowths- 

 of cryptoinicrocline. 



Qua rt z-phenocrysts are never as numerous as the felspars, and may 

 be entirely absent ; usually subhedral, they are sometimes rounded. 

 Hounded grains of quartz occur in LXXVI, S.E. 56. LXXVI r 

 S.E. 54 a yields subhedral quartzes; in ' K.N.W., Meldon B,' 

 from the collection of the late K. N. Worth (from the 71-15 

 exposure) some of the quartzes are subhedral, some are rounded ; 

 both are traversed by planes along which cavities have formed : the 

 larger of these cavities show fluid and bubbles, some of the smaller 

 appear to contain no fluid. Narrow veins of secondary quartz, 

 sometimes with a little red-brown mica, are found in many 

 slides. 



Mica is practically always present, characteristically of a rich 

 warm sepia, it is sometimes pale green (in contact with aplite, in 

 the quarry west of the Kedaven). Always in very minute form, its 

 quantity varies greatly from place to place, and even in the same 

 slide. It is this mineral that gives colour to the rock. That it 

 is to some extent secondary is certain, it invades, inclusions of 

 shale, tills vesicles in the one vesicular inclusion seen, replaces 

 quartz in casts of radiolaria, and takes part with quartz in filling 

 veins. That it is wholly secondary is possible, but not certain. 

 Pleochroic halos are frequent, but so small are the flakes of mica 

 that one halo will frequently extend to parts of several adjacent 

 flakes. 



Tourmaline occurs in small quantity, but probably is generally 

 distributed; it is so far rare that, unless several sections are pre- 

 pared from each exposure, it may not be seen. A little bluish 

 tourmaline occurs in ' K.N.W., Meldon B,' taken near LXXVI, 

 S.E. 15. LXXVI, S.E. 25 also yields blue tourmaline, as does 



