Георги Бончев: Разлика между версии

* {{cite book |last= |first= |chapter=Les houilles en Bulgarie par M le Professeur Dr. G. Bontchew |title= The Coal Resources of the World: An Inquiry Made upon the Initiative of the Executive Committee of the XII International Geological Congress, Canada, 1913, With the Assistance of Geological Surveys and Mining Geologists of Different Countries|year= 1913 |volume= II |publisher= Morang & Co. Limited|place= Toronto, Canada|pages= 747-761|url= https://archive.org/stream/cihm_80307#page/n391/mode/2up |accessdate= 30 май 2018}}