i 5 4 NATURAL HISTORY 
when we find the like declination in the Jlrata , I fhould think we 
may conclude, by parity of reafon, that there has been a like failure 
of what fupported them, in proportion to that declination. 
That we may trace thefe dippings through their feveral ftages, 
let us fuppofe A and B (Plate XVII. Fig. vi.) two Jlrata of clay 
or ftone under the furface, N ; if the ground gives way below, 
from R Q^to S I, thefe jlrata fhall fink into the pofition of C D; 
a declivity often feen on the fea-fhores and the banks of rivers : if 
the fubfidence be greater, reaching to L O, making with the hori- 
zon an angle of 45 ° (as in many fteep fides of vallies we may ob- 
ferve) then A B fhall defcend and become E F; but if the ground 
recedes at M P, as well as falls, an utter fubverfion enfues, and A B 
fhall become GH; that is, the Jlrata from horizontal become per- 
pendicular a : on the other hand, if the fubfidence under the jlrata 
A and B, be in the dire&ion of S, I, O, then fhall A B become 
T U, a pofition often to be feen in large inclined maffes of cliffs 
on the edge of the fea e . 
If the jlrata then, in which the lodes are found, have departed 
from their original pofition, it is no wonder that lodes fhould par- 
take of the fame alteration ; for whatever wrefted the Jlrata , muft 
have proportionably affected the contents of fuch Jlrata. Let us 
fee whether the fad; anfivers the theory. 
In the vicarage ground of St. Juft (Penwith) there is a tin-lode, 
to the north of which there is a valley, as C, Plate XVII. Fig. 11. 
accordingly the lode A B dips away towards this valley in the top, 
and underlies (as the tinners exprefs it) into the hill D ; that is, 
fhoots away from A to B. 
In a tin-mine in Rofmerguy cliff, in the parifti of Morvah, the 
lode near the brim of the cliff, A B, Plate XVII. Fig. hi. under- 
lies ten feet in fix perpendicular ; but this great and unufual dip- 
ping ( for, if the lode varies from a perpendicular five feet in twenty, 
we generally reckon it a great underlying) grows lefs 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 fame lode becomes perpendicular, as D F. 
This remarkable relation of inclined lodes to the next adjacent 
depreffion of the earth’s furface (which might be confirmed by 
many other inftances) will naturally lead us to conclude, that what- 
ever made the Jlrata fall fo much awry, muft alfo caufe every thing 
included in thofe Jlrata to fall proportionably. 
d This is the furprizing prefent fituation of the taken notice of by Mr. Hutchinfon, Traits, vol. I. 
ftrata of Caldy Ifland in Pembrokelhire, defervedly page 27, Edit. 2. e See OSH, Fig in. ib. 
Suppofe, 
