Beginning in 1707 and for virtually every year throughout the century, the minutes of the Presbyterian Church contain petitions from congregations and presbyteries pleading for ministers. At least every other year, the Synods of Philadelphia and New York wrote to presbyteries in Scotland or Ireland, begging for ministers to come to the New World. By 1740 there were 160 congregations; in 1761 the synod lamented: “The Church suffers greatly for want of a Opportunity to instruct Students in the Knowledge of Divinity.”
Between 1716 and 1766, some 200,000 Scotch-Irish immigrated, primarily from Ulster, with the majority settling in the Shenandoah Valley. The meeting of the first post-Revolutionary Presbyterian General Assembly, in 1789, counted 215 congregations with ministers and 204 without. Recognizing the shortage of ministers, the assembly called for each synod to recommend two members as missionaries to the frontier.
(From A Copious Fountain: A History of Union Presbyterian Seminary, 1812-2012 by William B. Sweetser Jr.)