The Geology amd PiiYyiooiiAriiY oe tue Lawnswood Ahea. 
71 
Under the miei'oseope an texture^ uuil'oriu ni cluuueier^ is 
seen. The pyroxene is a pale [)urplish, n()n-pleo('hroic type, subiiedral in 
I'orni, in grains up to 1.5 nun. diameter partially changed to green uralite 
on the borders. It is optically positive with very weak dispersion, twin- 
ning, 12V appi'oxiniately 40°, jS — 1.050, exiinetion A ^ — 51° — indi- 
cating pigeonitie dioj^s'ide. The [)lagioeIasc i& in euhediad laths (0.5 to 
1.0 ?nni. long) showing albite twimiiug and jnore rarely pericline twinning. 
This plagiocluse is perfectly iVesh labiadorite (Ab^^ with normal 
gradational zoning sometimes developed. llmenite occnirs in irregular 
opa.(inc areas up to one mm. in diamelm’, II ap])ears to have erj'stallised 
in the interstiees I'etween the earlier formed plagitndase and pyroxene 
grains. There is a pt'ripheral reaetion rim round the ilmenite of a brovcniah 
(diloritic malei’ial and chlorite stringcu'.s i)ass out into tlie plagioclase. 
(jluartz and ai)atite ai’e accessory minerals. Ai)proximate mineralogical 
composition is lal)radorite 50 ])er cent., })yroxene 40 per cent, ilmenite 
nine per c(ml., (juaifz one per cent. Tlu' rock is slightly uralitLsed quartz 
dolmate'. 
The hovdcu’s of the dykes are made up of line-grained uralitised dolerite 
allowing mon* advanced uralitisation tliau the central parts. ' In these 
Hiier-grained rocks the pyroxene is seen to he extensively uralitised to a 
greenish ami)hiboh> wliiidi is moderately pleochroic, X = light purplish 
colour, V green. Z ~ bluish green and the extinction is ai>proximately 
10°. Aiilie<lral jilates of strongly ph'ocliroic biotitc arc seen with X = 
liulit hrown, V and Z — yvv\ <lai'k hrownisli gr(*en. The ilmenite i^ 
partially leinu)xenisr'<l. Tliei'o art' occasional veinlefs of epidote. 
Order of crystallisation: — pyroxene and iahradorite, then ilmenite, and 
lastly (piartz (pi'imarv miiierahs), and, resulting from end phase liydro- 
thermal aefion and .subseipient movement betwaam dyke and country, bio- 
iite, epidotis leucoxei'c, and aifinolile. All the dykes ari‘ more uralitised 
at the edges because of siihsi'qucnt movement lietween dyke and countiw 
rock. Prider (22, pj). l()-48) has studied this phenomenon in more detail, 
at Armadalm The approximate composition is Iahradorite 48 per cent., 
pyi-owm* 25 per cemt., a<*tino]ite 15 [ler cent., ilmenite se^•en per cent., 
i|inirtz two ])er cent., hiotite and epidote three* p(*r cent. 
of the (finteiz dolerile.^. — These* hypahyssal, slightly uralitised- 
finai'lz dolei‘it<*s are thought to be pai't of a very widesiireael elyke* intru- 
sion throughout file Slate in Late Pre'-Camhriaii or Lower Gambrian time 
(24). The dykes were* inlnnh'd along lines f)f weakness caused by mihl dia- 
slrojdiism. . , 
A nai-row zone showing slight contact metamorphism — largely 
e'pidotisation of the country rocks — surrounds the dykes. 
V. Kcoxofijr (iKOLornu 
Re'fi'actory minerals and bandeal iron ore, in the south-Avest of the 
area are the* only min(*raU of economic im])ortance in the* area. 
A. HEFKArTORrKS. 
(i) Siliimanite, 
Refractory clays derived from the kaolinisation of siliimanite mica 
schists of the Jimpereling Sewies Inna* ])een quarried for the manufacture 
