How did these dark lenticular features form? (Near Hoganekkal Falls India.)
It's hard to tell (don't you have other photos?), but it looks like magma lenses.

The area is known for the Hogenakkal Carbonatite Complex, where lenses of carbonatite (a carbonate-rich magma) are found in pyroxenite dykes. See for instance this paper:


  The carbonatites form a series of discontinuous bodies within two nearly parallel NNE-SSW trending pyroxenite dykes intruding Precambrian chamockites. [...] Each carbonatite body consists of discrete veins and lenses emplaced in pyroxenite which has been permeated and soaked to various degrees by the carbonatites.


But it is hard to say if this is the geological formation that you saw without more information.