t 54 ' N A T U R A L H T S .T O
wfen- ive'find the'like declination.-^ the f t ’rOt'à, I Ihb^d*^hink' wè
may conclude^ by parity of reafon, that there has- been a like'failure
of 'what fupported them, in proportion to that declmatiohr
That we may trace ihefe dippings through'their feverar îtàges,
I e t% füppofe A and B (Plate-XV®.4 Fig. claÿ
cw *ftoneJ under the - ftirfaee,’ N W i f the ground’ ‘gives way below,
frôm R Q^tcfSTf théfè ftr à tâ flaall fink into1 the pofition of C D ;
à dechvity often féen on thefea-fh’ores and’ the banks 'of rivers: if
the ffihfidence be greater,“ teaching tojjPO, making1 with the hori-
zon ail angle o f 45 ° (as in many fteep fides o f vallièswe may ob-
ferve) then A B fliall defeend and become E F ^ b u t if the ground
recedes at M P, as well as falls, an utter fubverfion enfues, arid A'*®
fhall become G II ; that is, the ftrata from hd^ ilàa l' b8^®^''perpendicular
d : on the other hand, if the fitbfiderice under the ftrata
A and B, be in the diredion of S ,T , O, then fliall A B ’Become
T U, a pofition often to be feett ih large inclined maffes of cliffs
on the edge o f the fèâ*. ■
I f the ftrata then, in which the lôdes are found, have dfeparted
from their original pofition, it is ho wonder that lodes môurçppar-
take of the fame alteration ; for whatever wrefted the^^i'», muift
lave proportionably affeded the contents ô f füch fttatà. ' ’’Lei us
fee whether the fed anlwers the theory. '
In the vicarage ground o f St. Juft (Penwith) there is atin-lod'e,
to the north of which there is a valley^ as C, Plate XVIÈ Pig. n.
accordingly the lode A B dips' away towards this in the top,
and undéalîes (as thé timers exprefs it) into khé^hill D ; that is,
ifhoots away from A to B.
In a tin-mine in Rofmerguy cliffy in the patifh of Morvah,J the
lode near the brim of the eliff, A B, Plate XVII. fKgPrn. underlies
ten feet in fix perpendicular ; ' but this great 'and' umifual dipping
(for, if the lode varies from a perpendicular five feet in twenty,
we generally reckon it a great underlying) grows Ms and lefs, that
is, the lode approaches more and more towards a perpendicular line
as it runs farther into the hill, fo that when you come inwards
from the cliff about one hundred fathom, and as deep as C D,
Fig. in. (above which the ground is level and champaign, as M L)
the lame lode becomes perpendicular, as D F.
This remarkable relation of inclined lodes to the next adjacent
deprefiion o f the earth’s furface (which might be confirmed by
many other inftances) will naturally lead us to conclude, that whatever
made fat ftrata fall fo much awry, muft alfo caufe every thing
included in thofe ftrata to fall proportionably.
* This is the furprizing prefent fituation of the taken notice ojf by Mr. Hutchinfon, Traéfcs, vol.X
ftrata of Caldy Ifland in Pembrokeshire, defervedly page 27, Edit. 2. • See O S H, Fig* tn.ib.
Suppofe,
O F G O R JgJ J f j p L L. \ ' -i55
H 3u.ppcsfe, fcff inftance, TW W.&jFfg. ir.--Plate XVII.) a: finall
ifland with four lodes A B, G H, IK , N O , running eaft and weft
tfiifough it. ;In the middle, where, therebas been m> extraordinary
fecondary fubfidenee, (by which I mean a fubfidenee fines the firft
divifion of fo ]i^ lOTd?flmds;!atftlfe': time ofithe .creatiUh^the lode*
G H, ‘fhkll keepritst natural perpenHieulAr rdiredtion, and the Jhata
lie idrf';each ©therein", their. horizontal ufual manner j but fubfidpriceS .
and diflocations o f thety^#/&ohavifog- happened near- the lodges or
cliffs ciftthis ifland? J^as njay;fa&obfer ved mote.or Msahy^mbft all
cliffs)? the lodes which nin dehrdtjie extremities, lihllrbeifarioufly
affected. < Suppofe a lode, A -B, in i its,;Original perpendicular !pofb-
tion; a fubfidenee ’o f the^r^as happening underneath, th& ftratun&
E D, "finks and becomes D iF , thentdhdll th e lode, 'A D, fubfide-
$Lfo, and become C D, the inclination teiiding towards! the general
»Jail of the groundiat T ; b u t if-there bd.a^hbBeion thefotfftr' fide ,df
ithe ifland, as I K, ‘ whereUhe'bpping has .beenUn'a quite contr'ar|r
|diredtion, and the ftratum, K B,f dips fo&aA ito^becotmp K M, then
fhall the lode, '1 K, become inclined,, as^ X K. If farther on, and
iriearer th e " edge of :the cliff, at S,‘ there jbe a gredterkdip of life
sfche uhdet ftrata; as' from- O Q_to O R , 1 then yvill thd adjoining
lode,. N O, beconie- inclined, * as O P, an<J -fo . propbrtiopable *to
the dip ‘ df the ftrata^ fhall be the inclination o f lodes.
k Again : ^Vherei thfefe fubfidenbes' below’ were ’and sECT.vn.
have affgdted1 large tra£ts-t>f 'ground,\ there'is(a certain-uniformity
in thedlMlptaon^ ©f|^des j a^ for inftance, in fame parifhes in thfe in pardcuiw
Iweftern parts’ o f -Cornwall« (as-, in St. Juft) rv^e generally, .find the dillrias'
ilodes underlying towards the fouth; but ih the parilhes of Gulval
and Lannant, ’(the firft about feven,' the other ten miles frttm St.
Juft) they dip to the north as often as to the foiath, according as
-the ifuyidence prevailed. Where-ever the greateft fubfideiice was
.^to the north of fiich tradfcs, the tops of the Itidds wnujd crinfetjuently
»teint to the north, (as Plate XVII. Fig, hi. A B t<? C$) hid thfe
•lodes themfelves underlye to the fouth } and- this uhiformity would
take place, notwithftanding the lefler dippings of vallies and law
• grounds in fuch diftricS; for it muft be obferved, that vallies might
in fome meafure diminifli or vary, but could' not always wholly
controul or prevent the general'tendency of fuch large mafles falling
with fo powerful a momentum.
Again : Many vallies and depreflions were the confequences of sect vm.
the firft Settlement of folids, and'were therefore formed before the
Modes themfelves: it is no wonder therefore^if we fee lodfes inclining indinatioa
fometimes .without any regard to the declivity of fuch primeval
depreflions,