3. This act shall commence and be in force from and after the passing thereof. As at present bounded, the old channel at the mouth of Sugar Land run, at Lowe's Island,[3] is "the commencement of the line that separates Loudoun from Fairfax County and runs directly across the country to a point on the Bull Run branch of Occoquan River, about three eighths of a mile above Sudley Springs, in Prince William County." The Bull Run then forms the boundary between Loudoun and Prince William to its highest spring head in the Bull Run mountain, just below the Cool Spring Gap. The line then extends to the summit of the mountain, where the counties of Fauquier and Prince William corner. From the summit of this mountain, a direct line to a point[4] on the Blue Ridge, at Ashby's Gap, marks the boundary between Loudoun and Fauquier counties. A devious line, which follows in part the crests of the Blue Ridge until reaching the Potomac below Harpers Ferry, separates Loudoun from Clarke County, Virginia, and Jefferson County, West Virginia, on her western border. The Potomac then becomes the dividing line between Loudoun County, and Frederick and Montgomery counties, Maryland; "and that State, claiming the whole of the river, exercises jurisdiction over the islands as well as the river." [Footnote 3: "What is called Lowe's Island, at the mouth of Sugarland Run, was formerly an island, and made so by that run separating and part of it passing into the river by the present channel, while a part of it entered the river by what is now called the old channel. This old channel is now partially filled up, and only receives the waters of Sugarland Run in times of freshets. Occasionally when there is high water in the river the waters pass up the present channel of the run to the old channel, and then follow that to the river again. This old channel enters the river immediately west of the primordial range of rocks, that impinge so closely upon the river from here to Georgetown, forming as they do that series of falls known as Seneca Falls, the Great, and the Little Falls, making altogether a fall of 188 feet in less than 20 miles."--_Memoir of Loudoun_.] [Footnote 4: Designated in an old record as a "double-bodied poplar tree standing in or near the middle of the thoroughfare of Ashby's Gap on the top of the Blue Ridge." It succumbed to the ravages of time and fire while this work was in course of preparation.] This completes an outline of 109 miles, viz: 19 miles in company with Fairfax, 10 with Prince William, 17 with Fauquier, 26 with Clarke and Jefferson, and 37 miles along the Potomac. TOPOGRAPHY.