The networks offer up two keepers this week. God created the big-screen TV for just such spectacles as 30 Years of National Geographic Specials (NBC, Wed., Jan. 25,8 p.m. ET), a vivid testament to the diversity and splendor of our planet. The following night CBS devotes its entire schedule to In the Killing Fields of America Clan. 26,8 p.m. ET). Ed Bradley, Dan Rather and Mike Wallace report this stirring examination of the epidemic of violence in our cities.