Miocene

MIOCENE, in geology, The system of strata which occurs between the Oligocene and the Pliocene and thus forms the lower of the two divisions of the Neogene or newer Tertiary period. The term is derived from the Greek yEtoP, less, and Katvos, recent, and was introduced by Sir Charles Lyell. The name indi cates that the system has a smaller number of recent species than is found in the overlying Pliocene.

Conditions During the Miocene.

The close of the Oligo cene was marked by a very general regression of the sea. The beginning of the Neogene, that is of the Miocene, was marked by a widespread marine transgression, followed later by a general regression. Thus the whole Miocene corresponds to a cycle of sedimentation, quite distinct from the Pliocene cycle which fol lows it. As in the Older Tertiary period the marine transgression took place in Europe from three directions—from the North sea, the Atlantic and the Mediterranean. The Miocene deposits are found in basins connected with each of these three areas.

The North sea of Miocene times was never as extensive as that of Eocene or Oligocene times, it did not invade the Paris basin and did little more than cover the fringes of eastern England, northern Belgium, Holland and northern Germany. The fauna is entirely different from that of the Mediterranean and correla tion is difficult, especially as the North sea seems to have been cut off from the Atlantic by a ridge across the Straits of Dover.

The Atlantic washed the shores of France as it does to-day, and broad gulfs covered part of Brittany and Aquitaine, and there was a wide communication with the Mediterranean across the south of Spain.

The Mediterranean sea of Miocene times has left deposits which are the best known of all Miocene strata. The Alps had already been built up as far as general features are concerned, and the Miocene transgression was restricted to the surrounding peri-Alpine depression. From the Middle Miocene onwards the eastern Mediterranean was cut off from the western and also from the seas of India and the east. It became an inland sea with a specialized fauna.

Life of the Period.

From the beginning of the Miocene there is an entire difference between the faunas of the North sea and the Atlantic-Mediterranean, and it is difficult to give lists of characteristic fossils except for special areas. The fauna

of the Atlantic-Mediterranean area is obviously the parent of the existing Mediterranean fauna. Amongst foraminifera num mulites give place to Lepidocyclines, many of large size. Amongst fossils of zonal importance echinoderms such as Clypeaster, Scu tella and Echinolampas are especially abundant; amongst the numerous lamellibranchs and gastropods many species of Pecten are of very restricted vertical range. The important mammalian forms are noted below. For an account of the flora reference should be made to the article PALAEOBOTANY. It has been argued that the marked increase of herbivorous mammals in the Miocene, including Hipparion, was due to the spread of turf-forming grasses and that animal migration was greatly facilitated by the drying up of the Mediterranean.

Miocene Stratigraphy.

The following stages have been distinguished in the Mediterranean Miocene : The Burdigalian stage is here taken as the base of the Miocene, since reasons have been given for regarding the Aquitanian as the highest Oligocene. (See OLIGOCENE.) The Aquitanian marks the beginning of the Miocene transgression, but contains a number of characteristic marine shells which are not found in later beds. The Burdigalian (type area Bordelais) is marked by a conspicu ous transgression of the sea in the peri-Alpine region. Continental deposits of this age, notably the Orleanais sands, are marked by the appearance of such probroscideaus as Mastodon angustidens, M. turricensis, Dinotherium cuvieri, of Rhinoceros aurelianensis, Anchitherium, etc.

The Vindobonian stage (type in the Vienna basin) is marked by further local transgressions such as in Bas-Dauphine and in the enclosed Vienna Basin. The base of the stage, often of a sandy facies, has been designated the Helvetian (from the marine "mol lasse" of Switzerland), whilst at the top there are sometimes blue marls with a deep-water fauna of Pleurotomas—a facies dis tinguished as Tortonian (from Tortona, Piedmont). In the continental equivalents, such as the deposits of Sansan and Simorre (Gers) and La Grive-St.-Alban (Isere) the characteristic mammals are Mastodon angustidens, Dinotherium bavaricum, Rhinoceros sansaniensis, Dicrocerus elegans and the last Anchi therium.