of Mississippi and Louisiana 65 



tributed as to direction. These factors are on the whole quite fav- 

 orable to plant growth. 



Soil Temperatures 



A number of observations were made by means of a properly- 

 calibrated soil thermometer. Temperatures on the surface and at 

 a depth of 10 cm. were taken on clear days and indicate the 

 amount of heat to which the vegetation is exposed in ordinary 

 summer weather. The results are as follows : 



Surface of wet sand, 36 C. 



Wet sand, 10 cm. deep, 32 C. 



Surface of dry sand dune, 35. 5 ° C. 



Dry sand at 10 cm. 32 ° C. 



(Air temp, at same time, 30 C.) 



At a point near Port Eads, where some coal dust had been 

 mixed with the alluvium, and where some euphorbias and a few 

 other strictly radiate plants were growing, the surface temperature 

 was 50 C. The surface temperature of sun-baked alluvium at 



C. Here crotons, sidas and 

 some other plants were found plentifully. 



The temperature of the surface of a salt muck marsh was 

 38° C, and at a depth of 10 cm. 31 C. 



These observations, though scanty, indicate a high soil tem- 

 perature which probably often exceeds that observed. 



°-3 



General Physiographic Features 



The islands visited are of three distinct types : (1) The muck 

 marsh islands ; (2) The sand islands ; (3) The mud lumps. They 

 owe their origin to deposits of drift and alluvium from the Missis- 

 sippi River. Of the first group He a Pitre, Sundown, and Sam 

 Holmes Islands are types ; Ship, Cat, and Breton Islands belong 

 to the second type, while the third, the " mud lumps," consists of 

 a 'arge number of nameless islands just outside the several passes 

 which form the main outlets of the river. 



1. The Muck Marsh Islands. — These are near the outlet of 

 Lake Borgne, Cubits Gap, Thompson's Pass, and the numerous 

 bayous above the mouth of the river and are probably of the most 

 recent formation. They are low, level and mostly small, many of 



