The succession of Cambrian and Ordovician rocks in the area between Snowdon and Tremadoc is described. The Upper Cambrian includes Ffestiniog Beds with the Lingulella Band, and part of the Dolgelly Beds. The unfossiliferous Pennant Slates and Quartzites may be Upper Cambrian or Lower Ordovician. Many localities for Llanvirn graptolites are recorded. The Caradoc includes graptolitic slates below, and sediments and volcanic rocks with several shelly faunas above. The Snowdon Volcanic Series includes basalts and basaltic agglomerates. Some intrusions are related to the Snowdonian vulcanicity, others to the Caledonian deformation.