Mark fulfilled in his life what every Christian is called to do: proclaim to all people the Good News that is the source of salvation. In particular, Mark's way was by writing. Others may proclaim the Good News by music, drama, poetry or by teaching children around a family table.
There is very little in Mark that is not in the other Gospels—only four passages. One is: “...This is how it is with the kingdom of God; it is as if a man were to scatter seed on the land and would sleep and rise night and day and the seed would sprout and grow, he knows not how. Of its own accord the land yields fruit, first the blade, then the ear, then the full grain in the ear. And when the grain is ripe, he wields the sickle at once, for the harvest has come” (Mark 4:26-29).
http://www.catholic.org/saints/saint.php?saint_id=305
http://members.tripod.com/~shtyetz_john/life-of-st-mark-evangelist.html
http://www.communityofhopeinc.org/Prayer%20Pages/Saints/mark.html