Two distinct suites of dolerite, the Roraima intrusive suite (Precambrian) and the Minor Dyke suite (?Mesozoic) intrude the Guiana shield. The dolerites of the Roraima intrusive suite, which occur as major dykes, inclined sheets and sills, are tholeiitic in character and have been strongly differentiated. The series orthopyroxene-dolerite --> pigeonite-dolerite --> ferrodolerite --> leucogranophyre represents the main trend. Hornblende-granophyres represent heteromorphs of late pigeonite-dolerites and early ferrodolerites. Differentiation was accomplished mainly by crystal settling probably in the presence of convection currents; toward the close of crystallization a filter-press mechanism became operative. The Minor Dyke suite comprises numerous small dykes of undifferentiated dolerite. The intrusion of these dykes may be related to orogenic movements in the adjacent Andean geosyncline.