: 
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ye 
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24 BOULDERS. 
but that on the contrary, there was an interval 
between the granite formation, and the precipitation 
of the slate rock, here assumed to be, of the reputed 
primitive kind. 
The great difficulty involved by the occurrence 
of rolled pebbles or boulders in a district, is that 
of assigning them to that zra in geological events, 
to which they really belong, and though, in the pre- 
céding case of the granite boulder in slate-rock it 
is most easy to assume priority of occurrence to 
granite as a rock, relatively to slate as a deposit, 
there are instances in which doubts will occur, 
whether boulders of the same kind of rock might 
not be ascribed to a later revolution, or aqueous 
disturbance. Mr. Prideaux recognised Porphyritic 
boulders on Roborough Down, and quartz boulders 
on the slate tract between the rivers Tamar and 
Tavy. Now, the origin of these with respect to time 
must for the present at least, be doubtful. At the 
foot of St. Anne’s beacon, Cornwall, (granite, iron- 
stone, &c.) occur vast numbers of rolled pebbles, 
clay and sand in distinct beds, and, from the differ- 
ence in the phenomena of these deposits to those 
which we recognize as “ diluvium,” there is ground 
to believe, that they are of the same age as the 
granite pebble before spoken of,—referrible indeed, 
to what was probably the primitive flood or revo- 
lution, of which the earth as a solid body, presents 
us with any testimony. Certain it is, that in the 
south-west counties, instances occur of the granitic 
localities contributing rolled blocks of that substance 
to the districts immediately below and around them, 
but, the determination of the age of these 1s involved 
in doubt, from an equal possibility of their being 
due to the above-named primitive disturbance, and 
of their belonging to the flood more popularly allu- 
ded to. There is an exception however, to this 
species of hesitation, in favour of those pebbles found 
