122 BULLETIN: MUSEUM OF COMPAKATTVE ZOOLOGY. 



not raised beyond low-water mark, while where the surface of some of 

 the large heads is topped with dead coral or with worn masses of 

 the larger species, we may feel reasonably sure that the patch once 

 formed a part of an outlying patch of the elevated reef which has 

 been cut down to its present height by the combined action of the 

 sea and of atmospheric agencies. 



"We could not fail to notice how rapidly the water became clearer as 

 we steamed eastward toward the outer edge of the Barrier Eeef. Both 

 its color and purity were in great contrast to the almost turbid water 

 met with in the channel between the mainland and the most westerly of 

 the inner reef flats, where the silt from the numerous islands and the 

 wash from the mainland is thrown in very large quantities into the sea. 

 But while we could not fail to notice the great contrast in the purity 

 of the water, it may be due to its distance from shore, and perhaps also to 

 its greater depth allowing the silt to subside ; for the mud brought up at 

 our anchorage, only a third of a mile from the inner edge of the outer 

 reef flat, was not coral sand, as might have been expected, but mud 

 nearly as dark as that found in the inuer more westerly main channels. 



Returning to the inner reef patches of this part of the Great Barrier 

 Reef, we find Lark Reef, which forms the southwestern edge of Lark 

 Passage, a huge reef flat differing in no way from the inner reef flats 

 already described, except in the comparative absence of negro heads. 

 The patches on the north of the track to Lark Passage are compara- 

 tively small reef patches. On Swinger Reef, a small sand key has been 

 formed on it since the time of the last survey. A sand key will soon be 

 formed on the northern edge of Turtle Key (Plate XXXIII.) . 



On the east half of Marx's Reef, negro heads occur well on the reef 

 flat itself. Negro heads are also found on the reef flat to the west of 

 Marx's Reef. The only other reef flats we examined were those lying 

 to the westward of the Lizard Islands (Plates XYL, XYIL, XXXIV.). 



Eagle Islet is a low coral sand islet thrown up from the decomposed 

 beach rock found on the northwestern side of the islet. There is in the 

 extension of the islet a line of sandbars along the edge of Eagle Reef. 

 The summit of Eagle Islet is covered with a little low vegetation. As 

 far as we examined Eagle Reef flat, it appears to be made up of 

 decomposed beach rock sand (Plate XVII.). We could see but few 

 patches of this on the interior of the flat, only here and there an occa- 

 sional fragment covered with Alga3. Xear the edge of the Eagle Reef 

 fl:it, in from two to three feet of water at low tide, small patches of 

 corals began to make their appearance. The patches rapidly increased 



