SKEATS: CHEMICAL COMPOSITION OF LIMESTONES. 57 
given, but Dana recorded the remarkable fact that in some of the corals 
and coral rocks Silliman had found a considerable amount of magnesium 
carbonate. 
In the following year (1844), at the Washington meeting of the same 
Association,’ Dana again referred to the presence of magnesia in some 
of the corals, and announced the further discovery by Silliman of a 
certain amount of calcium phosphate in the specimens analyzed.  Silli- 
man’s early analyses were faulty, but he corrected them in a later 
article,” and also in the appendix to the report upon the United States 
Exploring Expedition, ‘ Zodphytes” (1846), p. 712. 
Silliman’s analyses included specimens of most of the reef-forming 
corals. Their hardness and specific gravity led Professor Dana to con- 
clude that the stony corals consisted of aragonite and not of calcite, — 
a conclusion which was subsequently confirmed by Gustav Rose and 
Dr. H. C. Sorby. 
The general result of Silliman’s analyses, showing the limits of varia- 
tion in the composition of the specimens, was given at the conclusion of 
his paper, and may be stated as follows : — 
Calcium carbonate . . . . . . . 90-97 per cent. 
Fluorides, phosphates, silicates . . . }2) “ & 
SreaMiownatter **.) cs Fo, ke Ge Be Te 
Silliman’s paper did not include his analyses of the coral rocks from 
the raised island of Makatea. These were given in Professor Dana’s 
work. The analysis of one specimen was as follows : Calcium carbonate, 
61.93 per cent, magnesium carbonate, 38.07 per cent ; another gave 5.29 
per cent of magnesium carbonate, while the corals themselves contained 
very little. In the same work Dana quoted analyses of coral sands, and 
showed that their composition was not different, as regards the magnesia, 
from that of the corals. The coral sands from the Straits of Balabac 
gave calcium carbonate, 98.26 per cent, magnesium carbonate, 1.38 per 
cent, alumina, 0.24 per cent, phosphoric acid and silica, a trace. Pro- 
fessor Dana quoted the analyses from Metia as an instance of dolomiti- 
zation during the consolidation of the rock beneath sea water, and it 
was upon the analyses from this island that he based his well-known 
theory of dolomitization. 
In 1847 a paper appeared in the Quarterly Journal of the Geological 
Society, written by the Rev. W. B. Clarke, on the Geology of the Island 
1 American Journal of Science and Arts, 1844, Vol. XLVIL., p. 135. 
2 American Journal of Science and Arts, 1846, 2nd Series, Vol. L., p. 189. 
8 Geology of the United States Exploring Expedition, J. D. Dana, 1849, p. 153. 
