ABSTRACT
Rocks of the midalkaline suite occupy the transitional field between the common rocks of the subalkaline suite and the rare rocks of the alkaline suite. The volcanic rocks and magmas of this suite typically range from basalt, via trachybasalt, basaltic trachyandesite and trachyandesite to trachyte or trachydacite. A perusal of the geological record reveals that a high proportion of all known midalkaline volcanic rocks were extruded during the Cenozoic Era, but even in this era they have a low abundance. R. A. Daly was the first to record that on many oceanic islands there was a noteworthy lack of midalkaline suite rocks of intermediate composition. The upper Cenozoic alkaline rocks of Kenya and northern Tanzania evolved concurrently with the development of the eastern branch of the East African rift system. The Eastern Australian Volcanic Province is an example of an extensive volcanic be1t that contains large volumes of midalkaline rocks.
