956 MINNESOTA BOTANICAL STUDIES. 



II. Shores of drift. 



A. Boulders and coarse gravel. 



1. With finer drift intermixed. 



2. With humus intermixed. 



B. Sand and fine pebbles. 



1. With slight intermixture of humus. 



2. With marked intermixture of humus. 



C. Clays with sand and pebbles. 



1. With slight intermixture of humus. 



2. With marked intermixture of humus. 

 III. Sh res of humus. 



A. Humus derived from aquatic and semi-aquatic vegeta- 



tion. Wet humus. 



B. Humus derived from land vegetation. Dry humus. 



C. Mixed humus. 



Besides these main types of shore there are, of course, to be 

 considered the very numerous varieties of mixed shores derived 

 from the combinations of two or more of the types givena bove, 

 over the same area. For example, intermixtures of talus and 

 boulders, talus, sand and humus, boulders and humus are alto- 

 gether frequent, and in walking but a short distance along the 

 shore it is common to encounter more than one or two types of 

 Substratum. Especially difficult to classify are the transitional 

 types between shores with rock in place and shores with talus. 

 This is particularly true of those shores such as are seen at 

 Windigo island, where the talus blocks are, many of them, sev- 

 eral feet, or even yards, in diameter. In such cases it becomes 

 evident that a certain sufficient size of talus blocks must afford 

 practically the conditions of rock in place. 



A chemical classification should perhaps be added to the 

 physiographic one which has been given, but the complexity of 

 the problems, owing to the great variety of rocks, are such as 

 to make only the more general classifications available or prac- 

 ticable. The shore lines may accordingly be denominated 



thus : 



1. Silicious shores. 



2. Aluminous shores. 



3. [Nitrogenous shore-*. 



The first and third are abundant, while the second is scarcely 

 to be found within the area of my observations. In the first 

 group are included the beaches and dunes, the talus or boul- 

 der shores, where the derivaties have been from silicious 

 granites, quartzites or gneisses, and the rock shores in which 

 silicious rocks are the prominent components. The third group 

 comprises the various types of humus soils and mixtures in 



