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The objeotB of thi.8 dlBGGi'tatlon are:
(1) To exmiïiio the mot ho de by w^iloh silio a
oah beoome ooroo&trateô by prooeoeee other than simple
mochanioal âialnlegratlon of pre«<existing rooks,
(2) To examine the main ocoarrenoaa of ohert
and flin t in the B ritish Isles in order to see which
of the varions methods outlined are important in ohert
formation. Foreign oxamg^les of ohert w ill only be
introduoed to illu stra te opaoial %)olnt@ since it is
fe lt that the B ritish examgjles are of a sufficiently
varied nature to oovor most of the prinoip&l types of
chert.
(3) By means of the evidence from the above
study to classify the B ritish cherts according to
origin.
O o S T i B B I f S
I . la g a o H ^E iB .
!a} Tb# m ttj» of abort.
b) file fern eni mote of ooearreooo of abort,
o) fbo f&etrlhotlon of abort.
n , m ft)tîoiggM Yiua o t
'#) dyngteetle eegregetiom.
b) jFaneoeutmiKixBneeoe eagragatlem i.
e) âpl0OD«tiv eogrogetione.
% n. ar w ii? M r n x f.
•) fhe eriglo of fU at.
b) fbo eriglB of Meaatola ebert otbart henf lin t.
a) The o rlg la o f Feleoaaole ec4 irro^Sasibriec ohert,
4) bummery o f prerlooB oonelwlome.
XT. m , <>?
e) The fbmet&oo of abort.
b) The oleealfioetton of abort,
a) The eXaeelfloatioB of Sritleh abort.
I. IHTHOJUaJ?IOlî
(a) Tho nature of chert.
Ohert oould be defined a tninutely-o%y@t^i 11 ine
Billoeoue rook, usually aseooiated with sedimentary reeks and
4
oompoBed predominantly of non-detrltal e ilio a , the ohert
having beoorao lith ifie d by the re d istrib u tio n and/or
crystftlllB a tion of opoline<*>eilioR or silica-*gel.
Tho name would therefore be applied to
(i) cilloeoua roekc; o f purely organic o rig in ,
( li) eilioeouB î'ooka of purely inorganic o rig in ,
( ill) Bllioooua rooks of mixed o rig in , both organic and
inorganic.
Chert may be formed %»
(i) from depoelts or concentrations of s ilic a produced at
the came time ae the enclosing stra ta ,
( li) by diagenetio changes some time a fte r the deposition
of the enclosing strota (i.e . i eneoontem, oreneously or la te r).
In general, therefore, a ij^a lify in g descriptive term
is neoeaaary fo r more precise d o fin itio n , e.g. when rich in
radiolg^&rian romains the ohert iw j be classed as a radiolarian
chert. The use of special names not involving the word chert,
however, based upon presumed origin ie to deprecated since
i t w ill be shown that the o rig in of much ohert ie by no means
no iToean©/
settled; wl.th an elaborate nomcnolatnr© the mme of many
ooourronoee would bo a matter of die put o. It ie therefore
profornblo to iiave a tom oovoriig a ll ooourronooa, mpable of
quail ft oat ion v;hon tho ixituro of tho ooourronce has been
eluoidatoC. This extended use o f the tern ohert is fortunately
current nrciotioo in both iSngland and Ameriea, although a few
npmoo fd r epoolal voriotloe have beoome ao firi/ily established
that to dron thorn would lead to oonfhsion* It is tho re f ore
unfortir ate tiia t ©uoh names Ijevo not always been used in the
same oonse, Tlse to m f lin t , fo r example, is used by Tarr to
dooorlb© a ll black or dark ooloured oherts. Othor authors
Imve attoRM’tod to distinguish flin t as being very fine grained
and roeeeesing a conohoidal fracture. Chert, on the other
hand, is ©aid to be coarser, breaking with a fla t sp lin te ry
fracture.
Tho American use of the term flin t is oontrary to
English uaage while d e fin itio n by means of firo tu re or grain-
l iG often unrolieblo. Tho type of fracture may depend
8 20
upon the method used to produce the fracture. Ory^^tocrystalline
cherts often have coarser patches which would have to be
oladsified aa inclusions of oliert in flin t if tho la tte r was
defined by its grain-oise. The term flin t is therefore best
uBcd in rathor a atrfvtigraphlcal sense, i.e . flin t Is that
0.
th a t/
ohert whloh oomirs in the ohall^ of 0.W, Europe, whether lig h t
or dark in colour, whether oryptocrystalline or
m ioroorystalline, whether possessirtg a oonchoidal or a fla t
fracture. This use of the term flin t is not therefore
s tric tly petrological although most flin ts as so defined w ill
he cryptocrystalline and possess a conchoidal fraoture.
Jasper is another term used to desoribo certain varieties of
ohert, namely the oimque highly-coloured va rie tie s, the
common colours being rod, brown or yollow. Presumably i t can
be applied to cherts of diverse orig in provided thBt they are
rich in highly-coloured Im purities such as iron-oxide. In
many cases the coloration may not have been an orig in a l
feature, being produced aubsequertly by the oxidation and
dehydration of e a rlie r in p u ritie s whloh wore not necessarily
highly- loured.
00
The term ohert has been arplied to certain
allioeous rocks found closely associated w ith lavas,
especially pillow -lavas. Chert has also been described as s
voln-m inoral in m ineral-lodes. In both oases there is
evidence that much of the s ilic a , i f not a ll, is of i:m@B»tic
o rig in . The term chert is , however, restricted to siliceous
rooks formed where contamination w ith scdimentaiy m aterials
is possible. Chert can therefore be used to describe siliceous
rooks found between discrete masses of lava where admixture
4.
admixture/
with the remainB of ailleooua organisme or d e trita l
m aterial oould liave ooourred. Chaloedonlo s llio a existing as
amygdales w ithin the lava is therefore not ohert.
m inutel^- crystal!ino
In the same way/sllioQ of magroatio o rig in replacing
tho walls of or fillin g fissures in sedimentary rooks may be
sailed ohert, p a rticu la rly when any oonneotion w ith a
magmatio souroo is not obvious,
Ohert and flin t are predo ainantly com osed of
orysta llin o e ilio a , the oommonest im purities being iron-oxide,
iron sulphides, rhombohedral carbonates and oarbonsoeous
m aterial. Some cherts contain considerable quantities of
calcium oarboiK te and may pass gradually into a surrounding
limestone. In the case of cli^rt ooourring in the coarser
sediments d e trita l m aterial such as quartz, calcareous
m aterial including sh ells, or glauconite may be present in
varying amounts, the ohert passing into what is best called a
oherty-sardctone, the chert acting as a cement. The
ifiacroatructura of certain rooks may be preserved in ohert.
The fhle e-bedding of olDstlo ecdlmonts may scxnetimos bo
observed while such features as e ilic ifie d oolites are not
uncommon. S ilio ifio d pellet-rooks have also been described.
The miorosoopioal examination of flin t shows that i t
is ty p ic a lly crypto c ry s ta llin e , the individual crystals of
s ilic a being extremely email. Between crossed niools the
5,
tho/
soot Ions give low aggrogato xjolsrlaatloio-ooloura except in
oertain ereaa wi:vero the 111-definecl oittlinea of small
organisms may be revealed by eomewh't ooarsor more b iré
fringent eiliO B, The iroeenoo of 0):)al has been oonsidered
lik e ly by many authors but te d iffic u lt to confirm. Between
crossed niool3 the presence of any isotropic s illo a is
d iffic u lt to detect. The p a rtia l s o lu b ility of flin t in
caustic potash solution cannot be used as eviderme since it
can be easily shown that the aesount dissolved depends upon the
time o f contact w ith the reagent. Tim elucidation of the
emot constitution of flin t must await fu rth e r work.
In most chert the individual crystals are uBimlXy
somewhat larger, often consisting of m inutely-granular quartz;
or m lm ztcly-flbrouc dial ce deny, the fibres being ty p ic a lly
arranged in short bundles w ith no special orientation. This
va rie ty of s ilic a has been called gramlo-chaloedony by Jes sop
to distinguish i t from ordinary fibro-cW lcedcny where the
bundles of fibres iiave a p a ra lle l arrangement.
The exact nature of chalcedony has long been a
matter fo r discm sion. There Is evidence that it is m%
m inutely fibrous quarts, e«g. its refractive index is consistent»
ly lower than quartz.
On the other hand l^rays roveel. same crystal
structure aa quartz. Furthermore, an intim ate
