66 Lloyd and Tracy : The insular Flora 



them being only a few acres in extent. They are composed of a 

 very fine black alluvium which is so soft that a pole or an oar can 

 be pushed into it one or two metres with very little exertion, and 

 the soil seems to be held in place principally by interlacing roots of t 

 maritime plants. The water between these islands is usually 

 shallow, seldom being over r.6 meters in depth, and always more 

 or less cloudy from the admixture of muddy water from the river. 



Oysters are very abundant in the brackish waters, and the is- 

 lands situated in the oyster region and exposed to violent wave- 

 action often have a beach from two to several meters in width, 

 formed of shells which are more or less broken. In all cases this 

 beach is found only on the seaward side of the islands, and always 

 resting on a foundation of black muck. In nearly all cases the 

 muck foundation is bare and plantless from 0.5 to 2 meters from 

 the water line ; then comes a rid^e of shells from two meters to 

 sometimes as much as twenty meters in width and from 3 dm. 

 to a meter high, the crest being near the river side, and the inner 

 slope very abrupt. These shell beaches are found only on the 

 shores of the muck marsh islands. 



2. The Sand Islands. — These islands are from five to fifteen 

 miles from the mainland, outside of Mississippi Sound, and prob- 

 ably owe their origin to the same causes which formed the muck 

 marsh islands, though they are now covered for the greater part 

 with sand. A close examination will usually show that a part of 

 each one has the black, mucky foundation of the marsh islands, 

 and if not found elsewhere some remains of this original soil can 

 usually be found on the southeast side of the island where it is 

 most exposed to erosion by winds and waves. All of these sand 

 islands are moving to the northwest, in the direction of" the pre- 

 vailing winds. The southeastern shore is usually abrupt, with 

 deep wdter within a few meters of the beach, while in nearly all 

 cases the western end of the island is a long and almost barren 

 sand spit, and often extending a mile or more farther in the form 

 of a sand bar which is uncovered at low tide. The sand covering 

 of these islands is only a few centimeters deep in some places and so 

 low as to be constantly wet, forming sand marshes, while in other 

 places the surface is raised from one to three meters, or even into 

 dunes twenty-five meters in height, as on Cat Island. Of course 



