272 SUCCESSION IN EURASIA. 



(1) series of sandy soil, (2) series of mixed sand and clay, (3) series of clay soil, 

 (4) series of saline soil, (5) series of humus soil. Many of these seem to be 

 societies and clans, or fragments of consocies typically developed elsewhere. 

 Most of them evidently stand in developmental relationship to each other, 

 though this is touched only incidentally. The first series exhibits the following 

 zones in sequence: (1) Eguisetum fluviatile, (2) Heleocharis pahtstris, (3) Carex 

 acuta, (4) Phalaris arundinacea, (5) Agropyrum repens, (6) Schedonarus inermis, 

 and (7) Heracleum sibiricum. 



Paulsen (1912 : 105) has traced the development of vegetation on the bare 

 sands of the Trans-Caspian lowlands, and has simmiarized his results as 

 follows: 



"If it has been thus established that the development of the sand-desert 

 has in all probability been from barchans to hummock-desert and desert- 

 plains, that it has proceeded from the most shifting condition to the more 

 stable, then we have at the same time traced the process of development of the 

 vegetation. This process is expressed by the order in which the various sand- 

 desert vegetations were described in the preceding pages, and it may be shortly 

 summed up: 



"1. Aristida pennata. 



" 2. Ammodendron, Calligonum, and other desert trees. A few herbs, mostly 

 annuals. 



"3. Desert-trees, smaller, but growing more closely. Several herbs, among 

 which perennials are dominant. Annual halophytes in the valleys. (Hum- 

 mock-desert.) 



"4. Small desert-trees (or none). No halophytes (or few). (Desert of the 

 Sand-plains.) 



"To this process of development the definition of formation by Moss might 

 be appUed. Even if the desert-plain be not closed, it is in itself stable, a 

 terminal sub-formation, and the definition given by Moss thus seems to be 

 applicable to the sand-desert as a whole. The definition of formation by Moss 

 applied in this way is employed here as a means of illustrating the unity of the 

 sand-desert and to elucidate its metamorphosis. It must be emphasized, 

 however, that I have grouped the different types of sandy desert in the same 

 formation not because they constitute what might be called a historical series 

 derived from each other in a definite sequence, but because at the present 

 time they agree on general lines as regards soil and growth-forms. More 

 thorough knowledge of the vegetations would perhaps lead one to designate 

 the different types of sandy desert as formations, or even to create more, e. g., 

 a special formation for the vegetation of the valleys between the sand-hills. 

 If one followed the concepts of Crampton in a recent paper, then the different 

 types would be regarded as formations; the barchan desert would be a migra- 

 tory (or neogeic) formation whose substratum 'owes its features to recent 

 geological processes,' whereas the desert of the sand-plains would be termed a 

 stable (or paleogeic) formation; presumably the hummock-desert would also 

 be a stable formation." 



MEDITERRANEAN REGION. 



Comes (1887 : 3) has sketched the details of the invasion on the lava-blocks 

 of Mount Vesuvius: 



The lava-stream, as it cools, fragments into blocks of various form and size, 

 the surface of which is usually alveolate as in pumice or converted into scoria. 

 The blocks weather readily on the surface, and the cracks and rough areas 



