The Alleghany Mountains, or that part of them with which we have now to do, stretch in three almost equidistant parallel ridges, from North-East to South-West, through the heart of Old Virginia. An occasional pass, or "Gap," through these ridges, affords communication, by good roads, between the enclosed parallel valleys and the Eastern part of that State. The Western of these Alleghany ridges bears the name of "Alleghany Mountains" proper; the Eastern is called the "Blue Ridge;" while the Middle Ridge, at its Northern end--which rests upon the Potomac, where that river sweeps through three parallel ridges almost at right angles to their own line of direction--is called the "Great North Mountain." The valley, between the Middle Ridge and the Blue Ridge, is known as the Shenandoah Valley, taking its name from the Shenandoah River, which, for more than one hundred miles, flows along the Western foot of the Blue Ridge, toward the North-East, until it empties into the Potomac, at Harper's Ferry. The Orange and Alexandria railroad runs from Alexandria,--on the opposite bank of the Potomac from Washington, and a few miles below the Capital,--in a general Southeasterly direction, to Culpepper Court-House; thence Southerly to Gordonsville, where it joins the Virginia Central--the Western branch of which runs thence through Charlotteville, Staunton, and Covington, across the ridges and valleys of the Alleghanies, while its Eastern branch, taking a general South-easterly direction, crosses the Richmond and Fredricksburg railroad at Hanover Junction, some twenty miles North of Richmond, and thence sweeps Southerly to the Rebel capital. It is along this Easterly branch of the Virginia Central that Rebel re-enforcements will be hurried to Beauregard, from Richmond to Gordonsville, and thence, by the Orange and Alexandria railroad, to Manassas Junction. Some twenty-five miles from Alexandria, a short railroad-feeder--which runs from Strasburg, in the Shenandoah Valley, through the Blue Ridge, at Manassas Gap, in an East-South-easterly direction--strikes the Alexandria and Orange railroad. The point of contact is Manassas Junction; and it is along this Manassas-Gap feeder that Johnston, with his Army at Winchester--some twenty miles North-North-East of Strasburg --expects, in case of attack by Patterson, to be re-enforced by Beauregard; or, in case the latter is assailed, to go to his assistance, after shaking off Patterson. This little link of railroad, known as the Manassas Gap railroad, is therefore an important factor in the game of War, now commencing in earnest; and it had, as we shall see, very much to do, not only with the advance of McDowell's Union Army upon Bull Run, but also with the result of the first pitched battle thereabout fought.