§2 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
200'. Third Terrace. — Section of a reef-forming cor] (Figure 2) 
showing the spicular structure cf the coral, the centers of calcification, 
and the thin mud-filled tubes of boring algz, which here and there 
have penetrated the substance of the coral. One cavity in the coral is 
filled with “mud” containing small organic fragments, while another is 
seen to have entirely recrystallized. Lining the walls of the cavity the 
spicular character of the coral seems to have determined that prismatic 
crystals of aragonite should be formed in crystallographic continuity with 
the coral fibers. The remainder of the cavity is filled with a mosaic of 
calcite crystals except round the tubes of alge, where long prismatic 
crystals, probably aragonite, have been formed. 
70’. Section of rubble below Second Terrace. One half of the 
section represents a rock made of organic fragments, coated with a 
deposit of fibrous calcite, cemented together with a mosaic of calcite. 
This rock was planed down, and on the planed surface a layer of in- 
crusting Polytrema grew, which has since recrystallized to clear calcite 
crystals. Above this the rock consists of a ‘‘ mud,” into which project 
from the surface of the Polytrema very large triangular-shaped crystals of 
calcite. The organisms include a radiolarian, two tunicate spicules en- 
closed by Polytrema planum and several perfect sections of Tinoporus. 
65'. Section of a reef-forming coral, some of whose cavities are 
empty, some filled with ‘‘mud” altering to aragonite, some with aragonite 
optically continuous with the coral fibers, while others are filled with a 
mosaic of calcite crystals. The difference between aragonite and calcite 
is well seen after boiling with cobalt nitrate. 
53’. A certain amount of incrusting fibrous calcite occurs, investing 
organic fragments. In some coral fragments aragonite has been formed, 
but most of the matrix and the material filling the cracks is calcite. The 
organisms include Halimeda, Lithothamnion, coral fragments and alcyo- 
narian spicules, and a number of small rounded mud pellets suggest in 
appearance the excrement of fish. 
18’. The matrix of the rock is chiefly “mud,” but some has crystal- 
lized in the form of calcite. Aragonite organisms, such as Halimeda 
and corals, remain unaltered, an secondary aragonite partly fills some 
cavities in the coral. 
15’. Largely made up of coral fragments with some Carpenteria, 
tunicate spicules and echinid spines. It was originally cemented by 
an investing layer of concentric fibrous calcite. Subsequent extensive 
solution has removed most of this layer and has in places dissolved 
part of ‘the organisms. 
