Kneeland.] 240 - [April 16, 
11. Ageregated grains, the first step in the conversion into rock, 
and differing from 1, chiefly in the greater proportion of phosphate 
and the less proportion of carbonate of lime. 
ur. A solid rock, the product of chemical changes, by which ear- 
bonate of lime is lost and phosphate of lime increased, in varying 
proportions as the specimens show on analysis. The carbonates are 
decomposed by heat and moisture, and with other soluble matters are 
removed by rain; the passage of the escaping gases through the mass, 
like the carbonic acid in the raising of dough which we bake into 
bread, renders the structure somewhat light and cellular, so as to re- 
semble and to be mistaken for a trachytic rock, of igneous or meta- 
morphic origin. In some cases the structure is so compact and hard 
as to resemble a feldspathic mineral or a weathered trap — even re- 
sembling the phosphate of lime rocks of the older crystalline forma- 
tions. Mr. A. A. Julien (Amer. Jour. of Science, Vol. 40, 1865,) 
has drawn attention to the rock guanos of the West Indies and the 
Eastern coast of South America, especially Sombrero Island, 18° 367 
N. and 62° 27’ W., where it occurs in veins through beds of lime- 
stone — with pseudomorphs rich in the bone phosphate, even 85 per 
cent., phosphoric being substituted for carbonic acid brought down 
by the rains into the joints of the limestone. These rocks also con- 
tain crystals of Brushite, containing 96.8 phosphate of lime — o¢ 
Metabrushite, 95 per cent., with one less equivalent of water — also 
two varieties, Zeugite with 85 per cent, and Ornithite with 94 percent. 
The varieties of all these rock guanos differ much in proportions of 
phosphates of lime. The oolitic or granular form, of many colors, 
contains both the bone or tribasic phosphate (3 CaO, PO,), and 
neutral bibasic phosphate (2 CaO, HO, PO,), with some phosphate 
of alumina, iron and manganese — evidently formed largely from, 
according to Dr. A. A. Hayes, fish bones and shells acted on by the 
acids of the guano — as stated below. 
The concretionary form, as in these specimens, has its upper sur- 
face covered with rounded elevations and nodules,.as in all compact 
calcareous concretions; this is white or yellowish or even reddish 
white, with a little carbonate and sulphate of lime, but rich in the 
bone phosphates. ‘The interior is full of cavities and irregularities, 
with a splintery fracture, and a hardness nearly equal to that of 
feldspar. ‘This is the more recent, bearing evidence in its cavities 
and agate-like lamination of the percolation and slow evaporation of 
waters ; there is no evidence of high temperature, great pressure, or 
See EE SO EE ee ee 
