Stromatactis is a spar network whose elements in cross section have flat to undulose lower surfaces and digitate upper surfaces. The network is composed principally of isopachous crusts of centripetal cement and commonly occurs embedded in finely crystalline limestone. It is the cement filling of interconnected cavities. Stromatactis of Upper Silurian red stromatactis limestone from Gaspé Peninsula, Québec Appalachians, exhibits two types of cements: (1) an isopachous cement that lined the walls of the conduits and is interpreted as early marine; and (2) a later blocky cement that occupies the centres of cavities. The first cement is composed exclusively of non-ferroan calcite, whereas the second cement is mixed non-ferroan and ferroan calcite. The early isopachous cement is white on polished slabs and has a turbid aspect under transmitted light. In a few samples, the relative homogeneity of this early cement is broken by the presence of distinctive grey clear calcite. Under cathodoluminscence, the grey clear calcite is non-luminescent and exhibits well defined bladed crystal shapes, whereas the white turbid cement has a dull orange luminescence and indistinct crystal shapes. The relationships between the two cements indicate that the dull luminescent cement is a secondary form of the non-luminescent cement, and it is concluded that the dull cement is the product of alteration of the non-luminescent cement by burial or meteoric fluids. The later blocky cement has the same dull luminescence as the white turbid cement and is thought to have been precipitated from the same fluids as those responsible for the alteration of the early marine cements. Oxygen isotopic values of the dull cement of the early isopachous crusts (mean δ18O= -6.8%o are intermediate between those of the non-luminescent early marine cement (mean δ18O= -5.3%o) and the dull luminescent blocky cement (mean δ18O= -11.8%o), while carbon isotopic values do not differ significantly (δ13C=+2.9, +2.4 and +2.6%o, respectively). The alteration also has affected the distribution of some trace elements, particularly Mg. Both unaltered and altered cements contain less than 1% microdolomite inclusions, but the Mg content of the background calcite of unaltered cement is three times that of altered cement (14171 vs. 5502 ppm). Precursor early marine cement is thought to have been low-Mg calcite. The mean δ18O value (− 5.3%o) of unaltered early marine cement is higher than values for the oxygen isotopic signature of Silurian oceans provided by brachiopod shells.