manufacturers), environmental (including municipal sanitation and water treatment facilities and flue gas treatment processes), metals (including steel producers), oil and gas services, roof shingle manufacturers and agriculture (including poultry and cattle feed producers) industries. We are headquartered in Dallas, Texas and operate lime and limestone plants and distribution facilities in Arkansas, Colorado, Louisiana, Missouri, Oklahoma and Texas through our wholly owned subsidiaries, Arkansas Lime Company, Colorado Lime Company, Texas Lime Company, U.S. Lime Company, U.S. Lime Company – Shreveport, U.S. Lime Company – St. Clair, ART Quarry TRS LLC (DBA Carthage Crushed Limestone) and U.S. Lime Company – Transportation. The lime and limestone operations represent our principal business.
On July 1, 2020, we acquired Carthage Crushed Limestone (“Carthage”), a limestone mining and production company located in Carthage, Missouri, for $8.4 million cash, subject to adjustment for working capital balances acquired. Carthage provides aggregate and pulverized limestone products that are used primarily in the agricultural, construction, roofing, and industrial industries. Carthage’s assets and liabilities are included on our September 30, 2020 balance sheet at fair values, with $7.5 million preliminarily allocated to property, plant and equipment. Carthage contributed $2.3 million to our revenues for the third quarter and the nine months ended September 30, 2020. We believe that this acquisition will complement our existing geographic footprint.
In addition to our lime and limestone operations, we hold natural gas interests through our wholly owned subsidiary, U.S. Lime Company – O & G, LLC. In the fourth quarter of 2019, we determined our natural gas interests did not reach any of the quantitative thresholds for a reportable segment. The revenues, gross profit and operating profit from our natural gas interests are included in Other for our reportable segment disclosures. Assets related to our natural gas interests, unallocated corporate assets, and cash items are included in Other identified assets. Segment disclosures for the three- and nine-month periods ended September 30, 2019 have been recast to be consistent with the 2020 presentation for each respective period.
Revenues increased 0.4% and decreased 0.5% in the third quarter and first nine months 2020, respectively, compared to the third quarter and first nine months 2019. Revenues from lime and limestone operations increased 0.5% and decreased 0.2% in the third quarter and first nine months 2020, respectively, compared to the comparable 2019 periods.
The increase in our lime and limestone revenues in the third quarter 2020, compared to the third quarter 2019, resulted primarily from an increase in sales to agriculture and roofing customers that was principally due to the addition of Carthage, partially offset by a 2.9% decrease in sales volumes for our lime and limestone products, primarily due to reduced demand from our oil and gas services, environmental, and construction customers. Third quarter 2020 revenues were also favorably impacted by a 3.4% increase in the average selling prices for our lime and limestone products.
The decrease in our lime and limestone revenues for the first nine months 2020, compared to the first nine months 2019, resulted primarily from a 4.5% decrease in sales volumes of our lime and limestone products, principally from the our oil and gas services, environmental, and steel customers, which was partially offset by increased demand from our construction customers, the additional sales provided by Carthage discussed above, and a 4.0% increase in the average selling prices for our lime and limestone products.
Gross profit increased 5.2% and 7.9% in the third quarter and first nine months 2020, respectively, compared to the third quarter and first nine months 2019. The increases in gross profit in the 2020 periods, compared to the comparable 2019 periods, resulted primarily from the increase in the average selling prices for our lime and limestone products, lower fuel costs, and increased operating efficiencies associated, in part, with the new kiln at our St. Clair facility, which began producing commercially saleable quicklime in the second quarter 2019, partially offset by increased costs incurred in the second and third quarters 2020 associated with responding to the COVID-19 pandemic.
The emergence of COVID-19 in the United States in the first quarter 2020 has created significant volatility, uncertainty and economic disruption to the general business environment. Federal, state, and local governmental responses to the COVID-19 pandemic, which include restrictions requiring social distancing and restrictions on business activities and movement of people in the markets for our lime and limestone products, began to take effect the last two weeks of March 2020. In the second quarter 2020, the pandemic and related restrictions on business activities resulted in a general economic slowdown, which has disproportionately impacted certain industries that purchase our lime and limestone products, including oil and gas services, environmental, and steel. In the third quarter 2020, business activity began to resume, and pandemic-related restrictions were eased; however, the recent surge of COVID-19 cases in the United States could impact the economic recovery that has occurred to date.