448 PROCEEDINGS OF THE ACADEMY OF [Aug., 



finally about 8 feet above the level of the lake, while during this 

 rise it thins rapidly down to about 9 inches in thickness. Throughout 

 this thinning it still maintains the same conformable relations with 

 the thickly bedded shales below and the thinly bedded shales above, 

 as were observed south of Staghorn Point. 



The reef now takes a sharp drop toward the north and thin layers 

 of corals and limy shales fan away from it (PI. X, PI. XI, fig. 1). 

 A- we approach lake level the mass of corals thickens, but it is soon 

 lost again in the complex of thin interbedded layers. We are here 

 evidently on the northern border of the southern reef. 



The Channel. — The space between the reef above described and 

 that which lies to the north was evidently an open channel during 

 most of the time when the corals of the two reefs flourished. The 

 abrupt descent of the fanning layers from both reefs toward mid- 

 channel and the discordance with the uniform southerly dip of the 

 shales which later overwhelmed the reefs point conclusively to 

 contemporaneous erosion for an explanation of the observed phenom- 

 ena. This channel was gradually filled with limy silt, and occa- 

 sionally an invasion of undersized corals ventured out into the 

 currents only to be stifled by more silt without attaining maturity. 

 These conditions are recorded by from 7 to 8 feet of thin limy shales 

 and interbedded colonies of scattered corals. Ripple marks and 

 cross-bedding in the limy layers bear witness to the shallowness as 

 well as to the motion of the water in this old channel. 



The Small Northern Reef. — At the southern edge of this small reef 

 the conditions observed at the northern margin of the larger southern 

 reef are duplicated. A thick mass of corals occurs near the lake 

 level. These corals rise rapidly and during their rise from the lake 

 the beds deposited in the old channel are seen to fan away from them. 

 The reef rises sharply to a height of 10 feet above the lake, and here 

 it is little more than a foot in thickness. Followed a short distance 

 north, the conditions on the southern edge are again met with, the 

 reef descends rapidly, thickens, and thin limy shales and coral 

 colonies fan away from the main mass. Just north of the reef border 

 this series of thin, ripple-marked, limy shales and seams of corals is 

 about 9 or 10 feet thick. These beds, which owe their origin to the 

 coral growths and to the disintegration of the corals, maintain their 

 character for some distance along the lake shore — that is, for some 



tance away from the reef (PI. XI, fig. 3), but, being reef -margin 

 dep< heir distinctive features disappear as the distance from 



