In spite of the fact that it appears on first view to be characterized by exceptional unity in form and contours, Spain offers in the in terior a great variefy of geographical condi tions; since nature has divided it into isolated regions essentially differing in climate, the geological composition of the soils, the flora, the fauna, and the character, customs and worldly possessions of its inhabitants. As a consequence of the exceptionally diverse geo graphical conditions, there are also marked differences in the density of the population of the several districts. Thus, from the perinhery to the centre the density of the population de creases in such a fashion that we can say the inhabitants are distributed in concentric zones of different density, the minimum, with the exception of the arbitrarily located capital, be ing at the centre, which is the part of Spain that nature favors least of all.
Geology.— Extending like a bond of union between Europe and Africa, the Iberian Peninsula makes the impression ,of being the least European of the three great Mediter ranean peninsulas: it announces Africa by the massive character of its forms and by the ex istence of its mighty central meseta. "It is,n says an authoritative writer in the study by R. Douville, entitled 'La Peninsule Iberique, on Espagne) (Heidelberg 1911), °a world apart, separated from Europe by the high Pyrenean barrier which cannot be surmounted easily except at the two extremities, separated from the Mediterranean world by the different coast ranges that now themselves form the shore and again leave only a narrow strip be tween themselves and the sea"— for example, the Valencian huerta and the plain of Tar ragon. It is indeed possible to give correct ideas of the main structural characteristics of Spain, but, as the same geologist cautions, we possess only relatively few precise notions con cerning the constitution of the zones of its mountain-folds, despite the fact that the stratigraphy of a large number of its natural regions is known in detail, and several of the regions have even become classics for the geological student. Thanks to the labors of Leon Bertrand and Bresson, of the Andalu sian commission, of Nickles and of R. Dou ville, we are beginning to know with some pre cision the structure of the two border chains, the Pyrenean and the Sub-betic or Penibetic. the latter including the Sierra Nevada. The complexity of the corrugations of the ancient rocks of the Cantabrian chain has been brought to light by Barrois, hut as to all that relates to the inequalities of the central portion of the meseta, grouped together by Choffat under the name of the Lusitano-Castilian System, we are still left in uncertainty: their structure and exact age are wholly unknown. Moreover, the Betic chain remains still unexplored, from the structural point of view.
With due recognition of the limitations just indicated we present the following gen eral and brief account, deduced from the facts actually known.
The orientation of the outcropping strata in the centre and northwest is different from that in the regions of the Pyrenean, Catalonian and Betic ranges. In the centre and north
west the outcropping strata— Silurian, for example — are aligned nearly southeast and northwest; in the Pyrenean Catalonian and Betic chains, on the contrary, the outcroppings in the main follow the direction of the moun tain ridges (chainons) and are oriented approxi mately southeast by east and north-northwest or southwest and northeast. Thus we are dealing with systems of corrugations or folds that are essentially different. The southeast-northwest folds are those of the mighty crystalline and Paleozoic mass of the Iberian meseta; the others are those of the bordering chains, the Pyrenean, Catalonian and Betic. These bor dering chains have a nucelus formed, like the meseta, of Crystalline and Paleozoic rocks, but in addition they possess representatives of al most entire sedimentary series. They belong to the zone of Alpine folds; they have had a geologic history extremely different from that ,of the meseta; and they also differ among themselves, one from the other, the Pyrenean and Catalonian chains showing distinctly special formations while the Betic chain shows types that are general and seen in the whole Mediterranean zone. The Catalonian chain strongly resembles in almost all respects the Pyrenean; but it probably formed a part, in some former epoch, of a mighty crystalline mass occupying a portion of the Mediterranean until submerged at an indeterminate period. Similarly, the Betic chain appears to have been partially submerged at the time of those Pliocene movements which accompanied the opening of he Strait of Gibraltar. The meseta is bordered on the south and on the north by two great transverse inequalities, of which the most striking is the fault of the Guadalquivir, at present underlined by the depression in which flows the Guadalquivir River: a gigantic fracture that cut, as with a knife, all the out cropping strata. A happening probably of the same kind was associated with the profound depression of the Ebro Valley. But the mesozoic deposits which, at different epochs, have overspread the meseta. now hide all the northeast part of the Guadalquivir fault and the fracture of the value of the Ebro. These deposits are found to be more horizontal as one proceeds farther toward the interior of the peninsula. On the borders of the meseta they are slightly corrugated and form the Sierra del Moncayo and the Montes Universales which mark out the northeast border. The Sierra Morena has no geologic individuality. The de pressions of the Guadalquivir and of the Ebro have been filled with Tertiary sediments; and the same holds true of the lowest portions of the meseta, the two Castiles. In consequence of a general uplifting movement throughout the meseta, with reference to the level of the sea, the Tertiary sediments upon the meseta have been raised until they form now the high plateaux already described.