The soil of this farm region is of a dark red color, owing to a friable shale which outcrops everywhere. The high, wooded bank of the Perkioming abounds in caves, scooped out by the hand of nature or man, as well as in great pits and shafts, for deep down under its shale, "Mill Grove" was rich in minerals, particularly the sulphide of lead, associated with copper and zinc, to reach which many excavations have been made. The lead mines of this farm are said to have been famous in Revolutionary times, and have been worked sporadically for a hundred years; if traditions are trustworthy, many a winged bullet that laid a Red-coat low in the War of Independence was a messenger from "Mill Grove." In some of the old conveyances, which go back to the time of Penn, the place was commonly designated as the "Mill Grove Mines Farm." It is recorded that the original tract of two thousand acres, extending from the Schuylkill to the Perkioming as far as the mouth of Skippack Creek, was sold to Tobias Collett by William Penn in 1699 for fifteen shillings. We shall soon see that the mineral wealth which "Mill Grove" was supposed to hide beneath its rugged slopes was a source of no little trouble to the Audubons, the Roziers, and their successors for many a year. At the foot of the declivity towards the west, half hidden by foliage, stood a picturesque stone mill, at a point where a solid rampart had been thrown across the stream to divert its power to the use of man. Hard by was the miller's house, which antedates the mansion, and which was built and first occupied by James Morgan, who came into possession of the property in 1749. It was this old mill site, originally distinct from the farm, that gave the name to the place. Behind the gristmill an extensive sawmill, built over the mill race, was also in operation. Today the dam is broken through, and the great mill wheel of wood and iron, twelve feet in diameter and fifteen feet wide, has come to rest after turning for more than a century.