The stratified formation presents a remarkable variety, almost all the systems being exemplified. The Archean, composed of gneiss and crystalline schists, and traversed by eruptive veins, extends over the greater part of the Eastern Rumelian plain, the Rilska Planina, Rhodope, and the adjacent ranges. North of the Balkans it appears only in the neighbourhood of Berkovitza. The other earlier Palaeozoic systems are wanting, but the Carboniferous appears in the western Balkans with a continental facies (Kulm). Here anthracitiferous coal is found in beds of argillite and sandstone. Red sandstone and conglomerate, representing the Permian system, appear especially around the basin of Sofia. Above these, in the western Balkans, are Mesozoic deposits, from the Trias to the upper Jurassic, also occurring in the central part of the range. The Cretaceous system, from the infra-Cretaceous Hauterivien to the Senonian, appears throughout the whole extent of Northern Bulgaria, from the summits of the Balkans to the Danube. Gosau beds are found on the southern declivity of the chain. Flysch, representing both the Cretaceous and Eocene systems, is widely distributed.
The Eocene, or older Tertiary, further appears with nummulitic formations on both sides of the eastern Balkans; the Oligocene only near the Black Sea coast at Burgas. Of the Neogene, or younger Tertiary, the Mediterranean, or earlier, stage appears near Pleven (Plevna) in the Leithakalk and Tegel forms, and between Varna and Burgas with beds of spaniodons, as in the Crimea; the Sarmatian stage in the plain of the Danube and in the districts of Silistria and Varna. A rich mammaliferous deposit (Hipparion, Rhinoceros, Dinotherium, Mastodon, etc.) of this period has been found near Mesemvria. Other Neogene strata occupy a more limited space. The Quaternary era is represented by the typical loess, which covers most of the Danubian plain; to its later epochs belong the alluvial deposits of the riparian districts with remains of the Ursus, Equus, etc., found in bone-caverns. Eruptive masses intrude in the Balkans and Sredna Gora, as well as in the Archean formation of the southern ranges, presenting granite, syenite, diorite, diabase, quartz-porphyry, melaphyre, liparite, trachyte, andesite, basalt, etc.
The mineral wealth of Bulgaria is considerable, although, with the exception of coal, it remains largely unexploited. The minerals which are commercially valuable include gold (found in small quantities), silver, graphite, galena, pyrite, marcasite, chalcosine, sphalerite, chalcopyrite, bornite, cuprite, hematite, limonite, ochre, chromite, magnetite, azurite, manganese, malachite, gypsum, etc. The combustibles are anthracitiferous coal, coal, "brown coal" and lignite. The lignite mines opened by the government at Pernik in 1891 yielded in 1904 142,000 tons. Coal beds have been discovered at Trevna and elsewhere. Thermal springs, mostly sulphureous, exist in forty-three localities along the southern slope of the Balkans, in Rhodope, and in the districts of Sofia and Kiustendil; maximum temperature at Zaparevo, near Dupnitza, 180.5° (Fahrenheit), at Sofia 118.4°. Many of these are frequented now, as in Roman times, owing to their valuable therapeutic qualities. The mineral springs on the north of the Balkans are, with one exception (Vrshetz, near Berkovitza), cold.