A similar development was rapidly taking place east of the Susquehanna, occasioned by the activity about Fort Augusta, at the Forks of the Susquehanna, and the Pennamite-Yankee War, which was being waged for possession of the territory in the Wyoming Valley and elsewhere, claimed by the Susquehanna Company of Connecticut, and the treaty of November 5, 1768, added much new territory. By an act passed March 21, 1772, the County of Northumberland was erected out of parts of the counties of Lancaster, Cumberland, Berks, Bedford and Northampton. The bounds of the new county stretched to the New York-Pennsylvania boundary line on the north and to the Allegheny River on the west, including in its extensive territory the present-day counties of Susquehanna, Lackawanna, Luzerne, Wyoming, Bradford, Sullivan, Columbia, Montour, Northumberland, Snyder, Union, Lycoming, Tioga, Potter, Clinton, Cameron, Elk, McKean, Forest, Jefferson, Clarion and parts of Schuylkill, Center, Mifflin, Juniata, Clearfield, Indiana, Armstrong, Venango and Warren. It is with eminent propriety this tenth county of Pennsylvania has been frequently styled “Old Mother Northumberland,” and each of her twenty-nine children refer back to her for their earliest political history. Its greatest proportions were attained in 1785, when, by the Act of April 9, all that part of the purchase of October 22, 1784, east of the Conewango Creek and Allegheny River was placed within its limits. The county thus extended along the northern line of the State as far west as the Conewango Creek, which crosses the New York-Pennsylvania boundary line in Warren County, and from the Lehigh River to the Allegheny River, with a maximum width of nearly two-thirds that of the State. The extent of this region exceeds that of several States of the Union. By the Act of September 24, 1788, Allegheny County was created, including all the territory in the State north and west of the Ohio and Allegheny Rivers, and from this territory, by act of March 12, 1800, the counties of Beaver, Butler, Mercer, Crawford, Erie, Warren, Venango and Armstrong were erected. Thus it would seem that the first five of these should be added with the offspring of Old Northumberland, for three years at least. If this be the case her children would number thirty-four of the sixty-seven counties of the State. The first curtailment of this generous domain resulted from the erection of Luzerne County, September 25, 1786. West of the Susquehanna the first county to which Northumberland contributed was Mifflin, erected September 19, 1789, but the part taken from Northumberland with additional territory from Northumberland and other counties, was erected into Center, February 13, 1800. The formation of Lycoming County, April 13, 1795, deprived Northumberland of the large extent of territory it had acquired under the purchase of 1784, with a considerable part of its original area.