(ieohKjjf (>i Lihjihde. ."{45 



rates. ■■ and is ctowiuiI hy tlir r]i|>fr Daciti' Series. (>). (Piute 

 XXX.. Fig. :i.) 



Coiupared with tlir Lnwfi- Seiies (rj. tlie roek is lighter in eolour. 

 more porphyritif. ainl thei-efoi-e more crystalline, and it contains 

 no fragmental material. It has fewer (juartz phenocrysts, but in 

 its earlier stages more felsjiai- (plagioclase heing greatly in excess 

 of orthoclase, whereas in series (<•) tlic iever.se is the case), and 

 ferro-magnesian minerals in about the same proportion. 



It is distinguished from the Upper Dacite (t) by its abundant 

 quartz and felspar phenocrysts, especially the formei-. 



Towards the top of the sei'ies. i.e., a}ipioai-hing its junttion with 

 the I'pper Dacite. the phenocrysts of ((uart/ and felspar become 

 fewer and much larger. The lock also becomes darker, and acquires 

 a certain outward resemblance to the I'pper Dacite. lint notwith- 

 standing this evidence of gradual differentiation within the series, 

 there is no dithculty, excepl where landslips have masked the 

 conditions, in drawintr a sharp boundary line with the I'pper 

 Dacite. The two series air quite distinct. 



St raf i graph iced Relaf ioimJii p. — Wherever this series is in con- 

 tact with the .series <\ the former occupies the higher level. 

 Wherever it is in contact with the Upper Dacite (e). it occupies 

 the lower level. Tliis general testimony of superposition indicates 

 that the order c. d . r . was tin- order i>i extrusion. For reasons 

 given later, the Upper Dacite must be regarded as the last of tlie 

 volcanic system, and the above older may be taken as confirmed. 



Fetroloyical Description. — The lower type is a gray rock, show- 

 ing very numerous ])lieiio( rysts of felspar, quartz, and biotite. 

 naming them in the order of decreasing abundance. The ])heno- 

 crysts consist of (|uait/.. idionioijihic. and often corroded; plagio- 

 clase (lal)ra<loiite chiefly i. /.onf<l. idiotiiorphic. though often frac- 

 tured, and .sometimes containing small inclusions of sphene ; ortho- 

 clase in clear rhombic crystals, e.xt ingiiishing diagonally; })lagio- 

 clase in excess or orthoclase; liiotite. He.xcd into conformity with the 

 flow line.s; and pyro.xeiie ( .' ) much altered, identified mainly l)y the 

 Ix)undaries, giving extinction angk's of 22 iU'^s- to '-V.) deg. (Jarnet 

 is common ; ajtatite and zircon not iinconimon ; ilmenite acces- 

 sory, rin' glassy ground-iiiass is obseuied by myriad fragments of 

 the phenocrysts. f laet ined Ity tlow movements, and now arranged 

 in coiifoiiiiity with the well-marked tlow lines. 



in the uppei- type of this series, quartz and felspar are in fewer 

 but larger phenocrysts; deeply zoned plagioclase is still in excess 

 of orthoclase; garnet is less abundant, if not al)sent ; biotite is 



