In the eastern hemisphere early in the Tertiary period a great submergence set in and marine waters spread over much of western and southern Europe, northern Africa, and southern Asia. The sites of the Himalayas, Alps, Pyrenees, Apennines and other mountains were then mostly submerged. A very remarkable marine deposit, made up almost wholly of carbonate of lime shells of a single-celled animal called Nummulites, formed on the floor of this vastly expanded early Tertiary mediterranean. This rock attains a thickness of several thousand feet. It is doubtful if any other single formation made up almost entirely of the shells of but one species is at once so widespread and thick. In the Alps this remarkable marine deposit may be seen 10,000 feet above sea level, and in Tibet fully 20,000 feet. Much of the rock in the Egyptian pyramids was quarried from this formation. Later in the Tertiary in Eurasia and Africa the marine waters gradually became very restricted, so that by the close of the period the relations of land and sea were not strikingly different from the present, although northwestern Europe, like northeastern North America, was notably higher just before the Ice Age than it is to-day. Eurasia witnessed tremendous crustal disturbances during the middle and later Tertiary time when, due to intense folding and uplift of great zones, the Himalayas, Caucasus, Alps, Pyrenees, Apennines, and other great ranges were formed. The crustal disturbance was most remarkable in the region of the Alps, where the movement resulted in "elevating and folding the Tertiary and older strata into overturned, recumbent, and nearly horizontal folds, and pushing the southern or Lepontine Alps about sixty miles (over a low angle fault fracture) to the northward into the Helvetic region. Erosion has since carved up these overthrust sheets, leaving remnants lying on foundations which belong to a more northern portion of the ancient (early Tertiary) sea. Most noted of these residuals of overthrust masses is the Matterhorn, a mighty mountain without roots, a stranger in a foreign geologic environment." (C. Schuchert.) The last period of geological time--the Quaternary--was ushered in by the spreading of vast sheets of ice over much of northern North America and northern Europe, and this ranks among the most interesting and remarkable events of known geological time. On first thought the former existence of such vast ice sheets seems unbelievable, but the Ice Age occurred so short a time ago that the records of the event are perfectly clear and conclusive. The fact of this great Ice Age was discovered by Louis Agassiz in 1837, and fully announced before the British Scientific Association in 1840. For some years the idea was opposed, especially by advocates of the so-called iceberg theory. Now, however, no important event of earth history is more firmly established, and no student of the subject ever questions the fact of the Quaternary Ice Age.