A. R. Horwood — Rock-soil and Plant Distribution. 9 



lu usiug the word rock-soil iu a special sense, to denote the soil 

 afforded by any particular rock or geological formation, ancient or 

 recent, by whatever agency formed or accumulated, it should be 

 pointed out that though there are, broadly speaking, quite fifty, if 

 not more, such types, yet the distinctive types of soil wliich exhibit 

 markeddifferences in composition, chemical and physical characteristics, 

 are briefly six in number : clay loamy sand, siliceous soils, limestone 

 or chalk, peat (raw or acid), humus (raw or acid), and saline soil. 

 Particularly is the presence of lime in a rock-soil a predominating 

 factor in plant dispersal, so that we speak of calcicole and calcifuge 

 species. The same applies to peat, humus, salt, and in a less degree 

 to clay, sand, and silica, since these last are liable to be frequently 

 mixed in varying proportions, and the existence of marls affords 

 a passage from arenaceous to calcareous types. 



If this is so it is obvious that the princi})les stated (akin to those 

 of AVilliam Smith) must have an important bearing upon geology, 

 especially in the possibility of determining thereby, in the field, 

 where pulaeontological data are wanting, the existence of definite 

 zones and subzones. The very vegetation which to the pure geologist 

 is the cause of obscuring the sequence, may in this way by ecological 

 methods be a means of determining their position, a paradox which, 

 however, is quite intelligible if one bears in mind the principles of 

 Smith that strata are characterized by their organic remains, more 

 particularly so as in the case of plant-bearing foimations, such as the 

 Coal-measures, it is possible to show tliat there was in the past, as in 

 the present, an ecological association of plants, that is to say, that dry 

 soil plants are found as they grew in certain associations, whilst wet 

 soil or aquatic types are founcl and grew in different positions, or at 

 different horizons, or can be found to have a different geographical 

 distribution. In particular such formations as the Trias (Keuper, 

 Rhsetics), the Lias, etc., have intermittent bands of limestone, skerries, 

 etc., which are not always to be seen in the field, in section or 

 otherwise, and in such cases there is often considerable difficulty, 

 especially in faulted regions or where drift obscures the solid rocks, 

 in tracing such beds across country. In the same way the existence 

 of irregular beds of sand in glacial deposits is often obscured by 

 vegetation or other causes, and tlieir existence, and their extent and 

 area, is often a pure matter of conjecture, frequently only to be 

 ascertained by aid of well-borings or other operations of the kind. As 

 the delineation of clay and sand, and the mapping of such details, 

 are of great importance, especially to the builder, agriculturist, and 

 horticulturist, and economic geology is to-day a great factor in survey 

 work, any accessory aids that can be rendered by the ecologist are 

 w^orthy of the consideration of geologists. And these are only a few 

 of the ways in which a study of rock-soil distribution of plants may 

 serve the geologist in good stead. Moreover, ecology has already 

 advanced so far that surveys upon a systematic and well-organized 

 plan have been made, and exact vegetation maps published, so that 

 it is hoped and ex!)ected that this science, wliich is of such importance 

 from the economic as well as the stratigraphic staiulpoint, aviU have 

 shortly to be established upon a national basis, and ecological surveys 



