92 NATURAL HISTORY 
nature left it in ; it is not found in ftrata or quarries but in de- 
tached angular maffes, fometimes in large rocks, and is ufually of 
a grey blueifh colour. If the nodules of this ftone are found of a 
portable lize and a plane furface, fo as to need little polifhing, they 
make ftones for grinding the moil precious colours, far beyond any 
marble and equal to any porphyry. 
There is another fort of Elvan, which confifts of a yellow clay 
cement, thick fet with opaque, white and yellow cryftalline gra- 
nules, and thefe thinly befprinkled with cinereous grains; both 
the grains and granules have fmooth and plane fur laces : This 
flone rifes in a quarry at Boreppa in Camborn, and elfewhere, is found 
in large nodules, immerfed five feet deep and more in the Vorlas 
clay-pits in Ludgvan, and in fmoothed nodules on the beach be- 
twixt Penzance and Marazion ; it is not near fo hard as the former. 
sect. iv. More common ftill, of more various appearance and neceflary 
Killas. u f Cj i s t Be ftone which we call Killas \ It is of the fchiftos kind, 
fome forts more friable, and fome more laminated than the reft ; 
there is fcarce any field or common, where in fome fhape or other 
we do not find thefe ftones, but where there are any quarries of 
it, the top of the ftratum is covered with loofe, thin ftones, inter- 
fperfed with earth and clay, and a few feet below, lies the folid ftone, 
the crevices of which are nearly perpendicular and horizontal ; thefe 
ftones generally dip towards the Weft, and rife flat, of a very even 
thicknefs, for which reafon I imagine they are called by the Tinners, 
Raze b ; they have a fmooth face for building, and make a 
ftrong wall, but are apt to be feather-edged, which makes them 
lodge water, and throw damps into the walls. There are three 
forts of this ftone, the yellow, the cinereous or blueifh, and the 
brown. The yellow is hard and lafting, if laid in nearly the fame 
figure as it rifes, but breaks eafily into fhivers, and acrofs the grain, 
fo that it will not bear hammering ; it does not ferment with aqua 
fortis , nor give fire with fteel, of a fandy grit and uniform texture, 
with a yellow ochreous clay in it’s commifiiires, and weighs to 
water as 2^ is to one. 
The blueifh killas is fometimes fo exceeding hard and ftubborn, 
that in the mine they give five pounds a fathom c for breaking it ; 
at other times it is as eafy to break as pitcoal. Round the town 
of Marazion, and other places, (but better ftill in a large and 
ancient quarry at Helfton in Kerrier,) there rifes a very tender 
killas, of the cinereous, and alfo of the yellow colour, both 
fprigged with fpecks of a darker hue, the texture of both the fame, 
* Dr. Woodward Cat. V. ii. pag. 6. fays we call 11 Woodward’s Cat. vol. i. pag. 202. m. 9. 
airy ftone killas that fplits with a grain. c Six feet long, fix high, and three wide. 
viz. 
