The formation of hydrothermal and limnic siliceous rocks, often called 'limnoquartzites' is also related to young Tertiary volcanic activity. The term 'limnoquartzite' is used in a rather loose sense encompassing postvolcanic siliceous rocks including hydrothermal silicites and opalates as well. The formation of these rocks can be more or less, directly related to the volcanic processes; e.g., the original volcanic rocks may be saturated by silica-rich solutions (silico-volcanites); sometimes substituting the original minerals or filling up of clefts and fissures (hydroquartzite). The silica-rich solutions reach the surface through hot-water springs and deposit siliceous matter there (geisirites) or accumulate in lakes where unicellular organisms with siliceous skeletal elements (Diatomaceae) can use the silica, first, to build their own shells, second, by the accumulation of these shells to cause deposition of the SiO2 material, in the form of opal and cryptocrystalline quartz. The bulk of the rocks classified under this name were in fact formed in a freshwater environment as demonstrated by the plant fossils quite frequently found in them. This group forms a very specific unit among the chipped stone raw materials in the Carpathian Basin. Its macroscopic features can be most varied even within a single source while different macroscopically similar types can be found at several localities within Hungary. The group can be separated into a number of specific (local) variants accordingly, but general limnoquartzite types can be found throughout the distribution area. The geological source distribution area in Hungary extends from the Danube Bend region until the Tokaj mountains. There are further occurrences from other regions of the intra-Carpathian volcanic arch - in Slovakia (Selmec and Körmöc Mts, Garam valley) as well as Transylvania (Nagybánya (Baia Mare) and Jószáshely (Iosasel) environs). Some regionally more or less meaningful groups can be separated among them like colourful-pastel varicoloured varieties typical of the Mátra foothill region and varieties rich in plant fossils, typical of the Tokaj mountains as well as a number of more general types which cannot be specifically located within the Carpathian volcanic arch. Subgroups can be defined on the basis of colour, texture, mineral composition and fossil content which can be allocated to specific source regions with more or less certainty. Analytical works devoted to the subject (BIRÓ-SZAKÁLL-SIMÁN 1984, BIRÓ 1986/b, VICZIÁN 1986, HAJÓS-KŐVÁRI 1986, NEWTON 1989) documented more heterogeneity, even within one source region. Wherever possible, sub-groups were indicated within the unified limnoquartzite group (Mátra, Tokaj region, Garam valley etc.). Analytical results, however do not support this sort of grouping as yet with a large set of representative data.