RE:KRAMER You are right again... I hated my geology classes at university... but i do remember for a fact that limestone can turn into marble when heated and under pressure... Let me do a little more research... Check at the bottom of that page: https://www.oakton.edu/user/4/billtong/eas100lab/metrxsum.htm Here it is: ' What happens to limestone when it is subjected to great heat and pressure? This occurs when limestone, for example, is subjected to heat and pressure and turns into a more coarsely-crystalline and sometimes banded rock called marble. The soft, clay-rich rock known as shale, when subjected to pressure becomes a harder rock called slate.'