76 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
corals. The rest of the section consists of clear dolomite, much of which 
is zoned. 
25’. The slide is cut to show a transverse section of a dolomitized 
coral — Figure VIII. — in which the inner and outer walls of the coral 
can be traced by “ dirt lines.” Many of the cavities between the septa 
are empty, except for a lining of dolomite crystals on the walls, but 
some are filled with a dark “mud” containing fragments of organisms. 
This “mud” filled the cavities in which it occurs before dolomitization 
took place. This is proved by its being in contact with the wall of the 
coral, and by the layer of dolomite crystals incrusting the “ mud” and not 
the coral wall. Staining with Lemberg’s solution shows that in this 
case the “mud” probably remains as calcite, though in some other rocks 
it appears to become dolomitized without apparent change. Quite dis- 
tinct from this dark “ mud” is a gray “silt,” which is found in the place 
of the stereoplasm of the coral. This material must have been formed 
after the dark “mud” was deposited, and after, or as a result of the 
breaking down of the stereoplasm of the coral. It sometimes occurs in 
corals preserved in calcite when its composition must be similar, some- 
times as in this case there is little doubt that it is dolomitic, as dolomite 
erystals are seen to have formed within it, and never in the dark “ mud” 
filling the interseptal spaces. 
Vatu Vara.— Vatu Vara or Hat Island is in latitude 17° 20! S., 
longitude 179° 30! W.! 
The island is one mile and a quarter in diameter, its summit is flat 
(consisting of a small reef), and falls off on each side in steep cliffs. It 
attains an altitude of 1030 feet, and from top to bottom is composed of 
limestone. Traces of five uplifts are visible on its ascent ; three of these 
are “terrace” formations, while two are represented by beach erosion 
lines. 
Calcium | Magnesium} Insoluble VATU VARA. 
Carbonate. | Carbonate. Residue. 
74.3 25.5 .20 2 
60.7 39.3 nis ZZ 
58.45 41.47 500 LL 
Chemical. — Only three specimens have reached me, and they are all 
dolomitic. The percentage of magnesium carbonate increases from 25.5 
at 1000 feet to 39.3 at 650 feet, while at 500 feet it reaches 41.47. 
1 Bull. Mus. Comp. Zool., 1899, Vol. X XXIII. 
