From An Illustrated History of the Commonwealth of Pennsylvania

The rise from 1840 to 1850 resulted from the iron (primarily) and lumber industries.  At the height of iron production, at least 55,000 tons per year were finished.  Furnace operators believed the supply of ore was inexhaustible.  There was a tremendous supply of charcoal and coak [sic].  They believed this would be the revival of post-Civil War industry.  Some worked with fire and clay for bricks and pottery.  Rich outcroppings of coal, part of the northern end of the Allegheny coal field, were mined primarily for local use.

Petroleum followed the lumber industry was as a major economic source.  Nearly 5,000 wells were drilled between 1870 and 1875 -- there had been only 5 wells in the Summer of 1870!  Forty-seven wells were drilled in January, 1876, producing 662 barrels.  The total oil production statewide in 1875 was 8,942,938 barrels; 7,621,479 of them were from Clarion, Butler, and Armstrong Counties.  An estimated 2,540,495 of those barrels were from Clarion County.