146 MINNESOTA BOTANICAL STUDIES. 



lower than the one above, The lower plane slopes gradually 

 to the surface of the water at medium low tide, ending in an 

 abrupt ledge about six feet high, which is exposed only at times 

 of very low tide. 



One or more pools from each plane were selected and studied. 

 The formation in which this series occurs may be best de- 

 scribed as a somewhat hard coarse sandstone, weathering in 

 rather irregular masses, plentifully sprinkled with bowlders or 

 concretions of a somewhat harder character. These concre- 

 tions seem to be largely responsible for the origin of many of 

 the pools of this locality. Becoming loosened in their beds, in 

 the cavity thus formed, they are used by the waves as chisels, 

 cutting out in the course of time pools several feet in depth and 

 diameter. In many of the pools these " chisels " are still actively 

 at work. 



Pool No. I. 



Location. — High tide pool. Situated on the summit of the 

 ridge about fifteen feet above the water at medium low tide : 

 Distant from the edge of the ridge twenty-five feet. 



Exposure to Tide. — On August 9th, during high tide, the 

 pool was entered but twice and then only by small dashes of 

 spray thrown up from below. 



On August nth, during a' squall at high tide it was filled 

 repeatedly in the same manner. The water entered from the 

 southwest. Drainage was to south and west. 



Shape and Dimensions. — Nearly circular. Greatest diam- 

 eter four and one half feet. Depth two and one half feet. 



Bottom quite smooth except for the presence of a few small 

 pebbles. Sides smooth. North side sloping. South side al- 

 most perpendicular except for a shelf of rock which extends 

 along the south and part of the east and west sides. 



Flo ra : Kin d and Distrib u tio n . 



1. Corallina aculeata. 



On the shelf on the south and on the sloping side on the north 

 extending across the bottom of the pool. All except a few 

 plants at the bottom appeared to be dead. 



2. Codium mucronatum. 



About twelve specimens were found on the west and south 

 side near the bottom. All of the upper ones were seemingly 

 dead. 



