CALCUATING FRACTION OF A REGION WITHIN AN IMAGE

["Mark Chan" <chanm(at)cadvision.com>]

Given:
An image consisting of two rock types. Rock type 1 is fragmented, dispersed
irregularly and embedded within rock type 2. Type 1 is generally dark brown
but not of unique color, type 2 color varies between black or dark brown in
color.

Want:
The fraction of type 1 as a percentage of total image.

Questions:
1) Any suggestion on the easiest way to do such computation?
2) Will some sort of image pre-processing help get the job done easier?
3) Any routine/program in existence I can utilize?

Thanks in advance,
Mark Chan