One notable occurrence pointing to a fairly comprehensive knowledge of geography of the Indian border by the Persians, was the voyage of the Greek Scylax of Caryanda down the Indus, and from its mouth to the Arabian Gulf, which was regarded by Herodotus as establishing the fact of a continuous sea. This voyage, or mission, which was undertaken by order of Darius who wished to know where the Indus had its outlet and "sent some ships" on a voyage of discovery, is most instructive. It is true that the accounts of it are most meagre, but such details as are given establish beyond a doubt that the expedition was practical and real. The Persian dominions then extended to the Indus, but there is no evidence that they ever extended beyond that river into the peninsula of India. The Indus of the Persian age was not the Indus of to-day, and its outlet to the sea presumably did not differ materially from that of the subsequent days of Alexander and Nearkos. Thanks to the careful investigations of the Bombay Survey Department, and the close attention which has been given to ancient landmarks by General Haig during the progress of his surveys, we know pretty certainly where the course of the Lower Indus must have been, and where both Scylax and Nearkos emerged into the Arabian Sea. The Indus delta of to-day covers an area of 10,000 square miles with 125 miles of coast-line, and it presents to us a huge alluvial tract which is everywhere furrowed by ancient river channels. Some of these are continuous through the delta, and can be traced far above it; others are traceable for only short distances. Without entering into details of the rate of progression in the formation of Delta (which can be gathered not only from the abandoned sites of towns once known as coast ports, but from actual observation from year to year), it may be safely assumed that the Indus of Alexander and Scylax emptied itself into the Ran of Kach, far to the south of its present debouchment. The volume of its waters was then augmented by at least one important river (the Saraswati), which, flowing from the Himalayas through what is now known as the Rajputana desert, was the source of widespread wealth and fertility to thousands of square miles where now there is nothing to be met with but sandy waste. As far as the Indus the Persian Empire is known to have extended, but no farther; and it was important to the military advisers of Darius that something should be known of the character of this boundary river.