The Contact Between tile (Jiunitic Hocks and the Cardcl^ Seiues 
AT ARilADALE. 41 
(2) Tlio pros{'iK'(' of idioblaslic Itmniuiliao in all tlu* slaty rocks of this 
firea. is intcrcstia^;' — tliis mineral may have been introdueed from j>'i*aiiitio 
intrusions, from the basic intrnsiv(\s (AjiTell, 11)39, p. 333, lias describtHl the 
development of dravitt^ in adinoles ainl (‘onsiders that the boron was intro- 
duced from nearby albiti' dohu’ites) oi' may have developed from the boron 
contained in the oripnal unmetainoriihosed sediments. (loldschmidt and 
l^eters (1932) ha\'e shown that the boron content of clay sedinumis is oftim 
sufficient to bring- about crystallisation of tourmaline when thest' s(*diments 
suffer dynamic metamor])hism and tliat it is not rn‘cessary to look to later 
acid intrusives for the origin of Hu* boron. The absence of tourmaline from 
the rocks immediately adjacimt to tlu* granite and its (auavrrence in the slates 
some distance away seems to suggest that it was not d(*rived from that 
source. It is possible tluai that tlu' tourmaline was introduced from the 
basic igiu'ous rocks but niori* probable that it r(*sulted from tlu* crystallisa- 
tion of original constituents of the argillaceous sediments from which the 
slates wert* ih'rived. 
(3) The devt'lopment of biotite aiul (diloritoid. Biotite has undoubtedly 
been formed dui’ing tbe iiKlamorphism of the slaty rocks as it is ol‘t(‘n W(*ll 
developed on the fradure (deacage sui’fac'es. It is well dt*\’eloi ed in small 
])ale brownish Hakes in the slates from tlie slal(' (juarry which an* (dost* to 
greenstom* intrusions but is alisent I'aidher to tlu* north (akmg the Hunbuiy 
Hoad) where tlu* slates ar(‘ some distance from the intrusive dykes, d’his 
]>oints to llu* de\(']opmenl of tlu* Inotite lu‘iug diu* to contacd alt(*ration c(m- 
secpu'nt upon the intrusion of the hasi<‘ rocks. Specinu'ns from tlie liaulage 
tunnel h‘ading out from the south-w<*st ('oriu*r of the slatf* (|uari‘y alforil 
information reganling the contact alteration (d‘ the (’ar<lup st*dim(*nts— 
tlie I’Oi'ks i»i this tuniie! consist of slates ami sandston(*s which liav(* luH'ii 
intruded by a <dilorite-albite epidorite sill and tlu'ii Iat(‘r by a (piartz- 
dolerite <lyk(*, Ixdh of which have in some Tiu'asure afieeded the sedinu'iits. 
The r()(dvs (together with brief dc'seriptions) enconnt(‘red in tliis tuiimd are 
(see tig. 4) 
From (*ast end to 4;") feet normal slates showing dev('lopmeiit of 
small flakes of pale hi'ownish biotile. 
From 15' to 49' in — -chlorif oi<l slate — this is the normal slate (simi- 
lar to that from 0' to 45') with a dev(‘lopment <d‘ small 
(diloritoid pori)hyrol>lasts, at lirst rare and tlu*n hi‘(*oming more 
abumlant towai’ds tlu* west. Tlie diloritoid porjibyroblasts are 
well formed })ul have been r(*]ilaced by penniiiit** with exlr(*mely 
weak biri'f'ringt'iuu* (almost isotropic' willi very weak ultra bliu* inter- 
ter(‘nc(‘ colours), ])osili\'e (‘longalion and marked jilemdiroism X 
pale yellow green, V - / d(*(*]i gi-ei'ii. At lirst sight tlu'se small 
porphyrohlasts appc'ar to lu* eddoritoid but tlu* pleoc'hroism and 
m-ieiilation are those of pcmniiiite. 'Tlu'rc* is little doubt, however, 
that they were orig-innlly (diloi'iloid. This is tlu* only jilacc* where 
this mineral has l)(‘en I'onml in the area and it is of interest as it 
sc'em.s to tix the ]U‘riod of intrusion of tlie chloriti'-alhif (' (‘pidiorite 
as more or less (‘onlemporam'ons with the earth movenu'nis affect- 
ing the (karduj) Sc'ries. (diloi’itoid is genei'ally regarded as a sti’{*ss 
mineral— in the prc'sent instance* it is (dt'arly rdat(‘d in scum* way 
to the epidiorite sill as it dc'c-n'iist's in amount away from that body. 
* All lueasureinentN aio iroiii liu* east end of the tiimu l uiul luive ]je( n iiuasured 
along tile north wall. 
