From the analysis of fossils, using the cathodoluminescence technique, it is seen that there is a pattern to the luminescence colours exhibited by dolomite, calcite and quartz, depending on the amount of the trace element Mn2+ present in the structure of the minerals. In the experiment dolomite luminesces turquoise blue, calcite luminesces pale pink & bright red, & quartz luminesces sea blue. The calcite and quartz colours agree well with the observation of both Agrell et al. (1965) and Sippel et al. (1965). However, the dolomite luminescence colour definitely agrees better with the colour described by Agrell et al. than with that observed by Sippel et al. Selective replacement of material occurs in Rugosa corals of the Devonian Onondaga formation of Southern Ontario. Quartz replacing material in the cavities of fossils is common, with the fringe wall of the fossil being of calcite as was previously observed by Middleton (1958). The matrix around the fossils consists mainly of quartz and calcite with minor dolomite. A systematic difference in style of replacement (with respect to the various heights from which the fossils came in the section) occurred when considering replacement of the fossils by quartz. Replacement of calcite by quartz followed by slight dolomitization of the matrix in the Onondaga is suggested by luminescence observation. Also suggested by luminescence observations is: 1) Dolomitization by a downward influx of Mg2+ ions, and 2) Homogenous and heterogenous micritic mud fillings in Rugosa corals' cavities. / Thesis / Bachelor of Science (BSc)