The series of good Emperors, from Vespasian to Marcus Aurelius (A. D. 70 to 181) formed military colonies of their veteran soldiers, whether German, Gallic or Roman, in the region originally inhabited by the Marcomanni. They were governed by Roman laws, and they paid a tithe, or tenth part, of their revenues to the Empire, whence this district was called the _Agri Decumates_, or Tithe-Lands. As it had no definite boundary towards the north and north-east, the settlements gradually extended to the Main, and at last included a triangular strip of territory extending from that river to the Rhine at Cologne. By this time the Romans had built, in their provinces of Rhætia, Noricum and Pannonia, south of the Danube, the cities of Augusta Vindelicorum, now Augsburg, and Vindobona, now Vienna, with another on the north bank of the Danube, where Ratisbon stands at present. From the last-named point to the Rhine at Cologne they built a stockade, protected by a deep ditch, to keep off the independent German tribes, even as they had built a wall across the north of England, to keep off the Picts and Scots. Traces of this line of defence are still to be seen. Another and shorter line, connecting the head-waters of the Main with the Lake of Constance, protected the territory on the east. Their frontier remained thus clearly defined for nearly two hundred years. On their side of the line they built fortresses and cities, which they connected by good highways, they introduced a better system of agriculture, established commercial intercourse, not only between their own provinces but also with the independent tribes, and thus extended the influence of their civilization. For the first time, fruit-trees were planted on German soil: the rich cloths and ornaments of Italy and the East, the arms and armor, the gold and silver, and the wines of the South, soon found a market within the German territory; while the horses and cattle, furs and down, smoked beef and honey of the Germans, the fish of their streams, and the radishes and asparagus raised on the Rhine, were sent to Rome in exchange for those luxuries. Wherever the Romans discovered a healing spring, as at Baden-Baden, Aix-la-Chapelle and Spa, they built splendid baths; where they found ores or marble in the mountains, they established mines or hewed columns for their temples, and the native tribes were thus taught the unsuspected riches of their own land. [Sidenote: 150. THE ROMAN FRONTIER.]