How to find the range and mean of each features over the four GLCMs..?

Hi, I have calculated the four GLCMs corresponding to four different directions (0,45.90,135) and distance one(d=1). Then I have derived 13 features from the each GLCMs. Now I want to find the mean and range for the each features over the four GLCMs such that my feature vector will be of size 26 (13 mean and 13 for range)But I don't know how to find the range for each feature over the four GLCMs. Please help me to solve this problem