Harker diagram is considered to be fundamental in the study of magmatic fractionation. However, the conclusions drawn from the diagram are dependant on basic geological facts instead of on the diagram itself. Fractionation trend of magmas demonstrated on Harker diagram is derived from the study of layered intrusions formed from basaltic magmas, and hence Harker diagram is not suitable for the study of granitic rocks. Many conclusions based on Harker diagrams of granitic rocks are not consistent with the basic geological facts.