A theory of the general circulation which admits mid-ocean fronts is e
xamined. This study is related to some earlier studies of two layer pl
anetary geostrophic systems, where it was argued that interior discont
inuities, or ''fronts'', generally arise as a result of the nonlineari
ty in the continuity equation. Similar dynamics are studied here under
more general circumstances. It is argued that the most general soluti
on for the interior thermocline requires several fronts, each of which
arises in response to a mismatch of subsurface potential vorticity st
ructure on isopycnals. Further, these fronts can influence each other
and determine large areas of the basin scale potential vorticity field
. Comparisons between special analytical solutions and numerical solut
ions are made. The main result of this work is that the frontogenisis
in planetary geostrophic dynamics is robust, characteristic of highly
stratified systems and of potential importance to basin scale structur
e.