Over its existence , the administrative county lost territory , both to the existing county boroughs , and also due to the municipal borough of West Hartlepool becoming a county borough in 1902 and Darlington in 1915 .
Over its existence , the administrative county lost territory , both to the existing county boroughs , and also due to the creation of county boroughs at West Hartlepool in 1902 and Darlington in 1915 .