This region lies in the northern part of the Tanganyika Gregory Rift Valley. Amongst certain aspects which are considered in less detail are the questions of tectonics and age, the rocks of the basement complex, and the geology of Lake Natron.
The main work consists of the systematic petrography of a group of Older Extrusives, andesitic, and basaltic rocks, and of a series of Younger Extrusives, soda-rich alkaline rocks, including the carbonatites of the volcano, Kerimasi.
The former occurred after a period of rift faulting; they are associated with larger volcanoes, and cover greater areas than the probable Upper Pleistocene Younger Extrusives.
Utilizing 12 new chemical analyses of rocks, 2 partial analyses, and incorporating all additional known rock analyses, a regional magmatic trend is outlined, with a parent magma of basic-andesitic composition, believed to have been derived from an olivine-basalt magma. This parent magma exhibits differentiation by fractional crystallisation together with alteration through contamination with the basement rocks.
To the east, on the volcanoes of Ketumbaine and Gelai, the magmatic trend is essentially, basic-andesite, andesite, Mawenzi-trachyandesite, phonolite, phonolitic-trachyte, trachyte.
Nepheline-melabasalt, nepheline-andesite, mugearite, and nephelinitic-phonolites were also noted. Similar rocks were mapped on the rift wall to the west.
After further rift-faulting, the nephelinitic-phonolite magma was further contaminated with the basement rocks, as shown by the fenites and tveitasites noted on the active volcano of Oldonyo L'Engai. Moreover, the nephelinitic-phonolite magma was desilicated by the assimilation of basement dolomitic limestone, and carbonatite was formed. Subsequently the source of the limestone having become exhausted, the parent magma returned to near its pre-desilication composition.
A detailed geological map has been constructed, and photomicrographs have been made of slides of most of the rock types.