Dysgeogenous and Eugeogenous Strata.-As may be gathered. from what has been stated in the chapter devoted to Geology, the rocks of the different periods in North Yorkshire are, for each as a whole, characterised by well-marked lithological peculiarities; and from this circumstance arises the fact that the tracts which they respectively underlie are found to possess tangibly marked physico-geographical peculiarities. To the question of the differences in mechanical constitution which the various strata present and the results which may be traced as springing from such differences as their primary cause it will therefore be needful to devote a few pages before proceeding further.

*

Two principal types of rock in respect of mechanical constitution may be traced. Following the nomenclature of the elaborate treatise upon this subject of M. Thurmann I propose to call them Eugeogenous (plentiful-detritus-bearing) and Dysgeogenous (sparing-detritus-yielding), these terms being better calculated than any which have been previously employed which I can call to mind to keep prominently in view what is essential. to be remembered.

The essential differences between the rocks of the two kinds are mainly in respect of their hardness, their power of absorbing and retaining moisture in small masses, their permeability when we consider them as forming extensive strata, and the differences in their power of yielding detritus which result from these characteristics. If we arrange the different kinds of rock in a scale according to their hardness and permeability, beginning with those which possess the smallest capacity for absorption.

* Essai de Phytostatique applique à la chaine du Jura par Jules Thurmann, Berne 1849, London, Williams and Norgate. See also a pamphlet of my own entitled "An attempt to classify the Flowering Plants and Ferns of Britain according to their Geognostic relations,” published in 1855.

and offer the greatest resistance to abrading influences, we shall have to place in the first rank the compact Limestones, Granites and Basalts; after them the compact Slaty rocks and Limestones like the Coralline Oolite and Calcareous Gritstone; then must follow some of the metamorphosed igneous rocks and the harder kinds of compact sandstone like the Brimham Grit and Kelloways band; then many of the Freestones, Flagstones and Chalks; and last of all and most absorbent of all are most of the rocks in which the argillaceous element is predominant, the Plates and Clays of the Carboniferous beds, the Shales and Clays of the Trias and the Lias, the Oxford Clays and Kimmeridge Clays of the Oolite. As regards permeability when bedded in extensive strata it is almost always the Limestones which are the most conspicuously traversed by fissures and breaks, and the softer argillaceous rocks which are the most compact, the sandstones and harder siliceo-aluminous beds occupying an intermediate position. The power which the different kinds of rock have of yielding detritus depends upon their position in the scale which has been indicated, and, especially when they form hill-masses and are permeable upon a grand scale, the soils which cover the dysgeogenous rocks are comparatively dry, whilst those which cover the eugeogenous rocks are comparatively humid.

We see that difference in respect of the characteristics which have just been noticed does not by any means run precisely parallel with difference in chemical composition. Calcareous rocks are not always dysgeogenous nor arenaceous and argillaceous rocks always eugeogenous, though such is most frequently the case. We must put the Limestones of the Carboniferous, Permian and Oolitic periods upon one side and place Basalt along with them; and on the other side Clays like those of the Lias and Triassic System, and Sandstones like those of the Trias, the Millstone Grit and the Lower Oolite, Chalk with the Slates and most of the Granitic rocks occupying an intermediate position. And a characteristic feature of our field of study and that which constitutes its special interest from the point of view

which we are now occupying is that in it the strata of the different periods not only underlie well-defined districts, but that they are composed either of rocks of well-marked eugeogenous type or of well-marked dysgeogenous type, or of bands of the two kinds mixed up together in the deposits of one period and that, as we have already seen, rocks of intermediate lithological constitution, as occupying large tracts of surface, are entirely absent.

Their Distribution throughout North Yorkshire.-The following table, then, is a repetition, except that the thickness of the beds. is omitted, of that which was given at the end of the chapter on Geology, each series being referred to its proper lithological type.

So that we have on the west bands of hard dry rock with thick eugeogenous interpolations between them forming the edges of most of the dales and ascending in some places to the loftier mountain summits, but more usually these summits and the higher surfaces of the moorlands, like their slope in the direction of the central valley, are formed of eugeogenous materials. Next follows a narrow strip of dysgeogenous and a broad area of eugeogenous lowland country, and on the east are two extensive tracts of moorland with contrasting lithological characters, both of which rise into the Middle Zone, and last of all we have on the south of them a eugeogenous valley, the proportion of eugeogenous surface to that of dysgeogenous being for North Yorkshire as a whole fully three to one.

Their Influence upon the Configuration of the Dales and Hillmasses. The results of these lithological differences are to be found, not only in the modification which they produce upon the general contour of the surface and the influence which they exercise in determining the shape of the dales and the configuration of the hill-masses, but they exercise also an influence which is by no means unimportant upon the topography of the vegetation. We will take the first question first and examine their influence upon a large scale and then treat the matter in its botanico-geographical bearings.

The different beds and bands of rock have all since the period of their original deposition been subjected to the influence. of energetic watery action. The glacial inundation must have reached a height of at least 1000 feet above the present sealevel, and that is only one flood amongst many. In almost all the dales the strata upon the opposite sides of the dale correspond to each other precisely. This correspondence is disturbed by faults in Teesdale, Lunedale and Arkengarthdale, and by smaller dislocations elsewhere, but as a rule our dales are dales of denudation. In Wensleydale we have the same bands of limestone in the fells upon both sides of the hollow with an excavation between them which often reaches a mile in width and a thousand feet in depth. In the dales of the Esk and Derwent districts the sandstones of the Lower Oolite may usually be seen above the shales of the Upper Lias upon both sides at an equal elevation above the stream. And we find that the general contour of the surface and configuration of the dales and coast is very much to be explained by the fact that the strata of different degrees of hardness have been unequally worn

away.

Both upon the east and in the west in the hill country two different types of scenery may be traced. The flat table lands of the limestone hills contrast conspicuously with the irregular undulations of the sandstone hills: the steep precipitous calcareous scars not less so with the irregular edges' of freestone

and gritstone. The sandstone hills are usually intersected by branching rivulets which flow from their upper levels gradually down their slopes into the low country: the limestone hills have neither streams nor natural pools upon their surfaces, but the glens slope suddenly and the water sinks through the calcareous beds to gush out in large volume when it reaches some less permeable stratum. The sandstone dales are open and irregular with gradual slopes and undulated embankments: the limestone dales are steep and narrow with sudden slopes and embankments rising up like a wall upon each side to shut them in.

It is in the east that the characteristic features of the hills of the two types are seen most readily. We have there two ranges of hills, one of which is fully 400 and the other 200 square miles in area, which throughout their extent are composed, the northern range of well-marked eugeogenous and the southern mass of well-marked dysgeogenous materials and most of the main branches of the Derwent rise amongst the sandstone hills and break through the limestone range before they enter into the low country. As they pass from one range to the other the change is so striking that it cannot fail to arrest the attention of the most casual observer. The difference in outline of the two kinds of hill may be well seen by looking up Bransdale or Farndale from the vale of Pickering. We have then immediately in front the flat plateaux and steep narrow dales of the calcareous range with its steep escarpment towards the north sweeping far away eastward and westward, and beyond rise the irregularly undulated masses of the heathery arenaceous moors with the high anticlinal ridge to bound the horizon. In the upper part of the dales the woods are scattered irregularly over their slopes and are more frequently to be found along the margins of the streams than anywhere else, but in their lower portion the steep calcareous embankments are usually covered. thickly with wood from the edge of the plateau all the way down the slope and the stream-side at the bottom of the dale left free. The Bran and the Dove are both partly swallowed up