INTRODUCTION This immense but little-known region of Venezuela merits in every respect a more detailed study than it has received in the past. The few people who have traversed the more accessible parts of the Guayana Shield realize that it offers tremendous economic potentialities, as well as many geological problems. Since so little has been published, especially in English, about the interesting Guiana region, this chapter, in addition to summarizing our knowledge of its geology, will also briefly outline its more important geographic features. LOCATION AND AREA The Venezuelan part of the Guayana Shield is located within the following approximate geographical limits: from 59°48’ to 67°52’ W. Long, and from 0°44’ to 8°35’ N. Lat. It is bounded on the north by the Rio Orinoco; on the west by the Rios Orinoco, Atabapo, Guainia, and Negro; on the south by the Sierras Parima and Pacaraima; and on the east by the broken mountain chain that extends from Mount Roraima to Cerro Venamo and by the Rios Venamo, Cuyuni, and Amacuro, as far as its contact with the Sierra de Imataca which forms its northeastern boundary. Geographically and geologically the area extends beyond political boundaries into all the neighboring territories. It encompasses an area of 421,900 square km, or more than 46 per cent of all Venezuela. Geologically this is the least-known part of the country, especially that portion west of 64° W. Long, which constitutes about 62 per cent of the total Shield area in Venezuela. GENERAL GEOLOGY The oldest