The stratigraphy and structure of the Cambrian and Ordovician rocks of the Blaenau Ffestiniog area are described briefly. This forms a background to a more intensive study of the four types of intrusive igneous rocks which are emplaced among the sedimentary and igneous rocks of the region. The intrusive rocks are: The Tan-y-Grisiau, Microgranite. Microgranodiorite and Quartz Latite. Dolerite. Multiple Intrusions. (Dolerite-quartz latite). The Tan-y-Grisiau microgranite is intruded into Cambrian rocks and has an extensive metamorphic aureole. Structural evidence indicates that it was emplaced prior to the imposition of the regional cleavage (Caledonian) and faunal evidence from outside the area under consideration shows that its emplacement occurred during post-Harnagian, pre-Longvillian times. The mineral assemblage in the main part of the intrusion (hornblende - biotite - plagioclase - orthoclase - quartz) can be equated, in terms of metamorphic facies, with a relict assemblage discovered in the hornfelses (andalusite- - Ooridrite - biotite - plagioclase - quartz). The north-western part of the microgranite has suffered extensive post-magmatic modification as a result of the operation of volatiles. Its mineral assemblage (chlorite - sericite - calcite - albite - quartz) can be equated with the ubiquitous retrograde assemblage in the hornfelses (chlorite - sericite - epidote - albite quartz). This assemblage is attributed to the presence of the slowly cooling microgranite and to the operation of volatiles derived from it rather than to retrogression brought about later by the imposition of cleavage or folding. The microgranodiorite and the quartz latites are considered to be closely related to the pyroclastic rocks of the Moelwyn Volcanic Series. They were intruded at very shallow depth and in some cases intrusion was continued after the rocks had consolidated. This gave rise to the development of autobreccias which have previously been interpretted as agglomerates. The amount of metamorphism associated with these intrusions is very slight. The dolerites occur both as thin dykes and sills of varying dimensions. They are petrographically identical to the dolerites which are ubiquitous throughout the Cambrian and Ordovician successions in the rest of North Wales. The multiple intrusions are thought to have been formed by rising dolerite magma remelting quartz latite through which it passed. This in turn was intruded into the centre part of certain dolerite sills. Petrographic evidence suggests that acid and basic magmas were held in juxtaposition at high temperatures without any significant mixing taking place.