

These above like previous image are Vulcanic bombs or Lapili.They are rare ocurences (Balcan,Italy and Brasil)in the nature and sign for existence in the distant past of big vulcano in the area.
In the Balcan area they are reaching 80 cm and 300 kg.Vulcan who have erupted these bombs in the past have very strong eruptive force and dating for this vulcan is around 30 milion years.Because of the eruptive force lava from vulcan have been erupted very high and cold air
have cooled down lava so the form have been allready formed splashing on the hot vulcanic ground have formed what we can see on the pictures posted from Ciko.
Now I can see reason for that in the geomorfology of the area.There are have been some previous post with pictures of Visoko area and the hill behind Visoko.That highher depresion between hils is probably vulcan opening.I can image that strong vulcanic rising have lifted lake sediments and explosion have cut Visocica hill from the crater forming structure like pyramid.This is my theory and I belive that these vulcanic bomb have been found in the areas of "erosion",wich is in fact two areas where lava have leaked the crater traveling towards the valey.
http://www.ngdir.ir/SiteLinks/Kids/Imag ... mb_fr6.jpg
http://www.beatrice-joya.exto.nl/galler ... 20318.html