CHAPTER XII. 
GEOLOGY— conimmZ. 
The Koaurite Series of Vizagapatam and Gaiijam. 
Vizagapatam district — Calcareous gneisses — The kodurite series — Relations 
to other crystallines — Igneous origin — Nomenclature — Mineralogy — Petrology — 
Magmatic differentiation — Localities — Occurrence in the Ganjam district — Chemical 
composition. 
Vizagapatam District. 
Lying to the east of the Eastern Ghats noticed above are the coastal 
plains of Vizagapatam, some 30 to 50 miles wide and not rising to any 
very great height above sea level. These plains are composed, as far as 
can be discovered from the scattered outcrops between the patches of 
alluvium, of a complex of Archaean crystalUnes of which the two main 
constituents are the gneissose granites and the khondaUtes noticed on 
page 241. These rocks often give rise to small tors and hills, respectively, 
protruding through the alluvium. Associated with the khondalites, 
usually at their junction with the gneisso;;e granites, are some very 
calcareous gneisses regarded by Walker as a part of 
The calcareous or the khondalite series and, Like the khondaUtes, 
a -gue ss . formed by the metamorphism of sediments. 
Mineralogically, these gneisses are of very variable composition, 
but are usually composed of pyroxene (macroscopicaUy dark to light 
green, microscopically light green to colourless ; probably diopside), 
wollastonite, scapolite, garnet (orange-brown macroscopicaUy, and Ught 
brown to yellow-brown microscopicaUy), calcite, and sphene. Microcline, 
orthoclase, plagioclase, quartz, and apatite, are less frequent constituents. 
It is difficult to find a name that can be used so as to include aU the 
varieties of these rocks. Thus the term scapolite-gneiss is not generally 
appUcable, because scapoUte is frequently absent. The most constant 
mineral is the pyroxene. But there are so many varieties of pyroxenic 
gneisses that it would be better to avoid this term if possible. Now 
it so happens that nearly aU the minerals in these gneisses contain an 
abundance of Ume, this richness in Ume being in fact the most character- 
istic feature in the chemical composition of these rocks. Hence I propose 
to refer to them as the calcareous gneisses or, for short, the calc-gneisses. 
