By looking at the map at the commencement of the volume, it will be seen that the Tigris and the Euphrates are parallel streams, flowing through the heart of the western part of Asia toward the southeast, and emptying into the Persian Gulf. The country between these two rivers, which was extremely populous and fertile, was called Mesopotamia. Darius had collected an immense army here. The various detachments filled all the plains of Mesopotamia. Alexander turned his course a little northward, intending to pass the River Euphrates at a famous ancient crossing at Thapsacus, which may be seen upon the map. When he arrived at this place he found a small Persian army there. They, however, retired as he approached. Alexander built two bridges across the river, and passed his army safely over.