 S. L. de Silva and P. Francis suggested in their 1991 book Volcanoes of the Central Andes that the Central Volcanic Zone should be subdivided into two systems of volcanoes , one in Peru and another in Chile , on the basis of the orientation ( northwest – southeast versus north – south ) . C.A. Wood , G. McLaughlin and P. Francis in a 1987 paper at the American Geophysical Union suggested instead a subdivision into nine different groups . 
