It
is often apparent, that the discharging operation had proceeded from a
centre, as some small matter may be perceived in that place. I know not
what species of substance this has been, whether saline or phlogistic,
but it must have had the power of either volatilising or changing the
ferruginous or red tinging substance so as to make it lose its colour.
I have only mentioned spherical spots for distinctness sake; but this
discharging operation is found diversifying those strata in various
ways, but always referable to the same or similar causes. Thus, in many
of the veins or natural cracks of those strata, we find the colour
discharged for a certain space within the strata; and we often see
several of those spots united, each of them having proceeded from its
own centre, and uniting where they approached. In the two veins above
mentioned, of whin-stone and spar traversing the strata, the colour of
the strata is, discharged more or less in the places contiguous with the
veins.
I am now to mention another appearance of a different kind. Those strata
of marl are in general not much consolidated; but among, them there
are sometimes found thin calcareous strata extremely consolidated,
consequently much divided by veins.
Pages:
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413