The response of a salinity-stratified fluid to sidewall heating is examined. Experiments were conducted using a linear salt gradient alone and with a combination of salt and thermal gradient in which the thermal stratification destabilized the density. The dominant feature of the flow was intrusions which propagated into the interior of the fluid. These were studied using shadowgraph, vertical temperature and salinity profiles, and dye streaks. Using the theory for convection in a long shallow cavity, an expression is derived for the velocity of propagation of the intrusions. The mechanism by which intrusions merge is also discussed to explain the observed formation of wavy interfaces during merging.