An isograd mapped within a single, homogeneous layer closely approaches the idealized concept of a line of equal grade. Correlation of an isograd from one area with one mapped in a widely separated area, can in rocks of similar composition be hazardous as each rock may have formed under different conditions at different times. A Finnish geologist, Eskola, was the first to attempt to correlate metamorphic zones between widely separated areas. Working in Finland, Eskola found that contact metamorphic assemblages were related to grade and composition. When he compared his results to others he found that some mineral assemblages were the same between areas, while in other areas in rocks of similar composition the assemblages were quite different.
Handout shows the relationship between metamorphic zones and metamorphic facies .
Not only for pelitic rocks as mapped by Barrow, but also for Basic and Calcerous rocks. If all the rock types in the table were interbedded correlation of the mineral assemblages within each, rock type would be very simple. Such a correlation, for several rock types within a single metamorphic grade, defines the metamorphic facies for that grade.