Complex sentence: The population of Ferryhill in 1841 was 854 and in 1901 had grown to just over 1,000, but by the time the two main collieries (Dean and Chapter Colliery and Mainsforth Colliery) opened this had swelled tenfold to 10,133 in 1911.
Split into simpler sentences: The population of Ferryhill in 1841 was 854, but by 1901 had grown to just over 1,000. By the time the two main collieries (Dean and Chapter Colliery and Mainsforth Colliery) opened this had swelled tenfold to 10,133 in 1911.