14 GENERAL HISTORICAL SUMMARY. 



years in the soil, provided they are not subject to the action of atmospheric 

 factors. 



p, "The alternance or alternative succession in the reproduction of plants, 

 especially when one forces them to live in societies, is a general law of nature, 

 a condition essential to their conservation and development. This law applies 

 equally to trees, shrubs, and undershrubs, controls the vegetation of social 

 plants, of artificial and natural prairies, of annual, biennial, or perennial species 

 living socially or even isolated. This theory, the basis of all good agriculture, 

 and reduced to a fact by the proved success of the rotation of crops, is a funda- 

 mental law imposed upon vegetation." 



Steenstrup, 18Ji2. — Steenstrup (1842 : 19) was the first student of peat-bogs 

 to turn his attention to the succession of fossil horizons preserved in the peat. 

 His pioneer work is the classic in this much-cultivated field, and since it is 

 practically inaccessible, a fairly fuU abstract of it is given here. The memoir 

 consists of five parts, viz, (1) Introduction; (2) Description of Vidnesdam 

 Moor; (3) Description of Lillemose Moor; (4) Comparative development of 

 Vidnesdam and Lillemose Moors; (5) General observations upon the Tree-, 

 Scrub-, and Heath-moors of Denmark. It is chiefly the detailed descriptions 

 and comparison of the moors which are summarized in the following pages: 



The'.bottom of Vidnesdam consists of a layer of bluish clay, containing leaves 

 of a grass and of Myriophyllum and fruits of Chara. Above this lies a layer of 

 fresh-water lime, inclosing a very large niunber of leaves of Potamogeton obtusi- 



FiQ. 1. — Section of Vidnesdam moor, ahowing various layers of 

 the ooaere. After Steenstrup. 



foUus zosterijolius, and perhaps of Sparganium natans. The leaves and stems 

 are incrusted with lime, and are stratified in this layer, in which Chara and 

 Myriophyllum also occur. An interruption in the formation of the lime layer 

 is^^indicated by a, lamina of Hypnum fluitans and Myriophyllum vertidllatum. 

 In the cross-section of the bottom of the moor (fig. 1), these three layers are 

 designated by m, n, and o respectively. The best series of layers, however, 

 is the marginal one, which follows the slopes all around the moor. The 

 drift c is covered by a layer of cones, needles, and branches of conifers, 1 to 1.5 

 feet in depth. In this are embedded large coniferous roots, the trunks of which 

 lie in the spongy peat layers toward the center. The large number of trunks 

 f oimd upon a small area leads to the conclusion that the pine (Pinus silvestris) 

 grew in a dense, piire stand. The pine trunks found in tMs layer r extend into 

 a layer of peat which lies directly above the lime layer n. The lower part of 

 the peat layer is filled with grass-like leaves, but the upper part consists wholly 

 of Sphagnum. Above, the latter is mixed with Hypnum cordifolium, which 

 finally becomes predominant and forms the layer q. The position of the 

 Sphagnum below and about the pine trunks indicates that this layer must 

 have been forming before as well as at the time of burial of the trees, while the 

 Hypnum layer must have developed subsequently. Pine roots also occur in 

 this layer, but the pines to which these stumps belonged must have grown 



