106 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
It is, therefore, considered essential that a limited number of field traverses 
should be made in each area to be mapped photogeologically. 
5. It is doubted whether a photogeological map of an area of Basement rocks 
can ever indicate the full metamorphic history of the area even after re 
connaissance field checks have been made. While migmatisation and meta 
somatism can, in suitable circumstances be inferred photogeologically, pro 
cesses akin to retrogressive metamorphism can only be recognised by an 
experienced field geologist and require detailed field work and the collection 
and microscopic examination of large numbers of rock specimens.