60 Puget Sound Marine Sta. Pub. Vol. 1, No. 9 



ing the several small islands and rocks which lie close by and are in- 

 cluded in this study. In some places it is very rocky, while in other 

 localities are found sandy or gravelly beaches (Plate 10). Most of 

 the rocky shore is composed of metamorphic rock, which weathers very 

 slowly and thus gives a very rough and irregular shore line. This is 

 specially conspicuous at Point Caution, Minnesota Reef, and almost 

 the whole of the west coast of the Island. In several places the shore is 

 composed of limestone ledges ; the largest of these are found at Lime- 

 stone Point near Roche Harbor, and at several places along the west 

 coast of the Island north of Deadman's Bay. Glacial boulders are not 

 uncommon along the shore, specially about Kanaka Bay, at False Bay 

 and near Cattle Point. 



There are several quite large bays along the coast which have rather 

 sandy or gravelly beaches. The largest of these are Griffin, False and 

 Westcott Bays. One of the largest gravel beaches is along the south 

 shore from Cattle Point westward. Here is an example of an eroding 

 shore about 2 miles long where waves from the strait are washing into 

 the gravel of Cattle Point Hill. There are also numerous small beaches 

 specially in little bays and coves. 



The constant motion of the tides through the channels between the 

 islands prevents stagnation of the waters along the shore. When the 

 tides change the water rushes very swiftly through the narrower chan- 

 nels and thus becomes well aerated. This is specially true of most of 

 the points along the shore of San Juan Island. Therefore, it is not 

 surprising to find a different algal flora at Cattle Point and Kanaka 

 Bay from that on the quieter shores near Friday Harbor. The interval 

 between highest flood tide and lowest ebb tide is about 4 meters, or 13 

 feet. This leaves some of the upper plants exposed much longer than 

 those growing lower. 



There are no large streams which enter the Sound in this vicinity, 

 and as there are no considerable streams on the islands, the surrounding 

 waters are free from sediment and fresh water. The absence of fresh- 

 water streams makes the salinity of the water fairly uniform about the 

 Island. At numerous places there are small springs from which the 

 water seeps out over the rocks along the shore and enters the salt water. 



The temperature of the water is fairly uniform in the different 

 places, and except for small tide pools, never gets very warm. The 

 winters are too mild for the salt water to freeze, and there is no floating 

 ice from the north to scrape the vegetation from the rocks. These 

 factors make it possible for the more temperate algal flora of the Puget 

 Sound region and many boreal species to find a common ground where 

 they can intermingle. 



