Radiolarites are used as environment indicators, but indicators of which environment? Moreover in literature, one commonly encounters opposition between calcareous and siliceous sediments. Is this correct for the initial environment of deposition ? Backtracking from the geological message to the biological signal of the initial environment requires a comprehensive knowledge of the different filters which have tangled it and of the actors which generate the signal. Siliceous plankton Radiolarians are planktonic organisms and as such obey the common rules of planktonic life: abundance/scarcity of nutrient. For decades one has evoked the necessity of a minimum silica content in sea water to allow these organisms to precipitate their skeleton, to develop and to be profuse. Consequently geologists have considered it logical to find abundant Radiolarians in rocks associated with ophiolites. However, the evidence to support this view, based on the study of Recent sediments, is lacking, indeed, Radiolarians are not most abundant along oceanic ridge areas but they are profuse in peripolar and equatorial belts and on some western sides of continents. These are all regions where the overall plankton levels (phyto- or zoo-) are prolific and where nutrient levels are high close to the water surface. There are no marked differences in the way oi life between siliceous and calcareous organisms. After death the only remnant of the organism is the test, as the cytoplasmic envelope soon disintegrates and exposes the mineral skeleton to the vagaries of nature. The dissolution of siliceous skeletons is more pronounced in surface waters. Moreover, the non-existence of a silica compensation depth explains how a present day Radiolarian can occur in sediment at 1000 m as well as at 10000 m water depth: the critical zone for its dissolution is within waters above c. 500 m. There is a ＇＇behavioural＇＇ difference between siliceous and calcareous organisms concerning their depth of dissolution. Siliceous sediments Siliceous skeletons are often very rare in sediments except in those deposited from high productivity zones. Indeed, if the original signal is reflected in the sediment, its variations are strongly enhanced and several times exaggerated. All occurrences indicate that the siliceous signal must be preserved beyond a critical threshold for tests to remain abundant. In moderate productivity zones for instance, 1/3 to 1/2 of the surface production reaches the ocean floor, while in zones of high productivity, 2/3 is deposited. On a global scale, it is generally estimated that less than 1 % of the biogenous silica produced is found in the geological record. Siliceous rocks: example of Mesozoic radiolarites Radioiarian tests are made of opal-A which is highly instable and is successively transformed into opal-CT and then into quartz. These two transformations occur in a liquid phase (one can here understand the risk that a Radiolarian with a delicate and fragile morphology will disappear before it reaches posterity) which explains both the relatively small number of well-preserved Radiolarians and the frequent concentration of silica as nodules. These transformations are associated with a considerable porosity reduction. This porosity variation is coupled with a major decompaction factor which has to be considered when reconstructions of sedimentary piles are put forward. One may generally estimate a multiplication of the present thickness by a factor of 5. Application of this factor to tethyan radiolarites leads to the following original thicknesses for cherts: 20 cm (mean present thickness: 4 cm, factor. 5) and for shares: 1 cm (mean present thickness: 0.5 cm, factor: 2). Using this formula, 100 m of radiolarites, a common thickness for this series in the Tethys, corresponds to an original thickness of approximately 450 m. The silica phase in most siliceous rocks is transformed from opal-A through opal-CT to quartz during progressive diagenesis. The transformations are principally controlled by temperature and time. This explains why quartz predominates in older rocks (Mesozoic and older) and opal-CT in the Cenozoic (porcelanites, tripoli,...). Host rock lithology, together with porewater chemistry, also plays an important role. The presence of primary carbonate tends to promote the opal-A to opal-CT transformation, whereas the presence of clay tends to hinder it. For a primary alternation with slight lithological variations, an onset of the opal-A to opal-CT transformation can be different, depending on the lithologies. The beds with an early onset of this transformation import silica from the beds in which the transformation is delayed or inhibited. Local redistribution of silica between the contrasting lithologies is possible during the opal-A to opal-CT transformation, whereas the redistribution seems more restricted during the opal-CT to quartz transformation. Field evidence also suggests the formation of chert nodules and pinch-and-swell beds by additional silica cementation within the opal-A zone, especially within calcareous siliceous rocks. Silica redistribution during the opal-A to opal-CT transformation enhances the variations in composition between originally clay-rich and clay-poor layers. The majority of siliceous rocks lose their porosity by mechanical and chemical compaction and reprecipitation associated with the opal-A to opal-CT transformation. One estimates that 30 to 40 My are necessary for a transformation from opal-A to quartz in zones with a high sedimentation rate and 60 to 70 My for those with a moderate sedimentation rate. For the tethyan radiolarite, the quartz stage was thus reached around the Lower Cretaceous which explains: (1) irregular levels because of incomplete lithification, only the porcellanite stage being reached during the Tithonian tectonic phase (Hellenides structures); (2) the difficulty in dating cherts associated with ophiolites of inner zones, which were tectonically deformed before the outer zones (i.e. Pindos-Olonos zone) where dating is relatively easier. Domains of deposition and age of radiolarites Radiolarites are Tethyan (those which are not of unknown origin !). They have been compared to the red clays of deep oceans. Current knowledge indicates that this comparison is not valid, their only common characteristic being their colour (their deposition below the CCD not being systematic). Were they deposited in large or small basins? The problem with large basins is that they have mostly disappeared. On the other hand some radiolarite were deposited in small basins. The important requirement is a strong upwelling. One notices that radiolarites were deposited during Liassic, Dogger-Malm and Albian-Cenomanian times in general and in some local regions, such as marginal basins of East Arabian platform (Hawasina, etc.), siliceous sedimentation occurred from the Permian to Late Cretaceous. One may compare these localities with the present Owen and Somalia basins, all places of active upwellings, but only partially influenced by fully open-marine conditions.