In
accordance with this mode of origin, it is exceedingly common
to find in the centre of all these nodules, both old and new,
some organic body, such as a bone, a shell, or a tooth, which
acted as the original nucleus of precipitation, and was thus
preserved in a shroud of mineral matter. Many nodules, it is
true, show no such nucleus; but it has been affirmed that all of
them can be shown, by appropriate microscopical investigation,
to have been formed round an original organic body to begin with
(Hawkins Johnson).
[Footnote 4: Apart from the occurrence or phosphate of lime in
actual beds in the stratified rocks, as in the Laurentian and
Silurian series, this salt may also occur disseminated through
the rock, when it can only be detected by chemical analysis. It
is interesting to note that Dr Hicks has recently proved the
occurrence of phosphate of lime in this disseminated form in
rocks as old as the Cambrian, and that in quantity quite equal to
what is generally found to be present in the later fossiliferous
rocks. This affords a chemical proof that animal life flourished
abundantly in the Cambrian seas.]
[Footnote 5: It has been maintained, indeed, that the phosphatic
nodules so largely worked for agricultural purposes, are in
themselves actual organic bodies or true fossils. In a few cases
this admits of demonstration, as it can be shown that the nodule
is simply an organism (such as a sponge) infiltrated with phosphate
of lime (Sollas); but there are many other cases in which no actual
structure has yet been shown to exist, and as to the true origin
of which it would be hazardous to offer a positive opinion.
Pages:
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79