4i6 



NATURE 



[Feb. 28. 1889 



vegetation of grasses with a number of other small plants, 

 their roots being matted together in an almost continuous 

 mass, possessing a certain degree of elasticity. During 

 the rainy seasons this stratum absorbs a considerable 

 quantity of water, getting thus much softer and ofiering 

 less resistance to the underlying loam and its downward 

 thrust. The whole mass is therefore in a state of plas- 

 ticity, and consequently a sliding motion begins more or 

 less as if it were a glacier. The slope not being uniform 

 in all its parts, nor the disintegration of the rock every- 

 where of the same depth and degree, it follows that the 

 sliding too will be unequal, and so an extraordinarily com- 

 plicated system of stresses and counter- stresses is de- 

 veloped, which of course causes the surface to take a 

 wrinkled or wavy appearance. I have tried in vain to 

 find numerical values for the limits of sloping which allow 

 of the formation of these surface-waves or ledges ; the 

 fact is that it depends to a considerable degree on the 

 interior conditions of the soil and subsoil, which are not 

 visible from outside. The lowest slope, however, that I 



have seen covered with ledges, was between 8° and 10°, 

 the steepest 45°. The total amount of sliding soil is 

 in some places far from being insignificant. I remem- 

 ber a locality in this neighbourhood, where there was 

 twenty-five years ago a shallow depression, in which 

 during each rainy season a small pool formed with Najas 

 microdon, A.Br., Wolffia Welwitschii, Hegelm., and 

 even Marsilea subangulata, A.Br. This depression has 

 gradually been filling up, and is now on the verge of 

 disappearing altogether, the material having been derived 

 from a grassy slope on its northern side, which is covered 

 by finely developed ledges. 



My observations refer to the valley of Caracas ; but 

 as identical causes must be at work in other coun- 

 tries, it appears to me that everywhere the formation 

 of ledges on mountain-slopes and hill-sides will probably 

 depend, first of all, on the conditions of the ground and its 

 vegetation, the action of earthworms being of secondary 

 importance. 



Caracas, January 6. A. ERNST. 



A MOVABLE ZOOLOGICAL STATION. 

 T N Bohemia, much attention has been given for more 

 ■*• than twenty years to the study of the fauna of ponds 

 and lakes, but the work has been rendered difficult by the 

 impossibility of the organisms being examined instantly 

 in their habitats. The transportation of the material a 

 long way has led to most of the finer objects being de- 



stroyed. Last year, a little movable station, suitable for 

 real biological work, constructed after a sketch drawn by 

 Dr. Ant. Fritsch, was presented by Mr. Ferdinand Perner 

 to the Committee for the Physical Exploration of Bohe- 

 mia ; and there is good reason to hope that the use of 

 this structure may be attended by important scientific 

 results. There is room (12 square metres) for from two to 

 four workers. The building consists of eighty pieces, the 



