MUSEUM OF COMPARATIVE ZOOLOGY. 157 



the reef, there are only one or two lines of breakers acting directly upon 

 the shore portion of the reef. 



« 



Dana has described the so-called modern chalk of Oahu, which is 

 found at a single locality near Diamond Head, in a part of the elevated 

 coral reef. It is at the foot of a tufa cone rising from the water's edge, 

 and, as Dana has akeady stated, coral must have been thriving on the 

 shores when the eruption took place, as there are fragments imbedded 

 in the tufa, although the chalk itself is of later origin. There is nothing 

 to be observed throwing any light on the causes which have produced 

 this chalk at this particular part of the elevated reef, except that it 

 must have been deposited in a confined area, subject to special condi- 

 tions. Yet this chalk is not more similar to the modern chalk than the 

 modern chalk dredged ofif Nuevitas, which was deposited under most dis- 

 similar circumstances. The Oahu chalk appears to differ very slightly 

 from such deposits of fine coral sand as are deposited in sheltered locali- 

 ties on the shores of coral beaches. It does not contain any organic 

 remains, but has in addition the peculiar fracture of chalk, and, as is 

 stated by Dana, is used on the blackboard in some of the schools of the 

 islands. 



At Makapuu the reef has been raised about twenty feet, and farther 

 north the whole coast is fringed with a growing reef, extending in some 

 places over three quarters of a mile in width. There are extensive sand 

 dunes, also, mentioned by Dana and Brigham, a short distance back of 

 the shore reef at the foot of Konahuanui. 



The elevated reefs of the Sandwich Islands, although not elevated 

 more than twenty to twenty-five feet, are extensively quarried as lime- 

 stone for building purposes, especially those parts of the reef which evi- 

 dently formed its inner portion, and in which the corals and mollusks 

 living on the surface of the reef have been admirably presei'ved. 



On the southern edge of the Aliapaakai basin, six miles west of 

 Honolulu and three quarters of a mile from the sea, there is a raised 

 coral reef which has been much displaced ; it has been fully described by 

 Dana. Living corals are comparatively rare upon the reef-flat. Large 

 specimens of Pontes ^ flourish in pits and hollows of the reef, and a 

 scanty marine fauna, with occasional masses of NuUipores and Sar- 

 gassum. The top of the edge of the reef is ban-en, and is deeply fur- 

 rowed ; it is only somewhat farther down the slope that the reef fauna 

 flourishes actively again. 



^ Dana speaks of the huge size of individual masses of Poritesin the rock of the 

 inner reef of Tongatabu, which were twenty-five feet in diameter. Geology of 

 U S. Exploring Expedition, 1849, p 39. 



