21-Quartzite

The white color of this rock is due to the purity of the original quartz sand grains and to the high degree of mineralogical sorting of the grains in their depositional environment, which probably was a marine beach. Pressures applied during later metamorphism forced the sand grains tightly together and caused them to recrystallize, giving the rock the "toughness" typical of quartzites. Rock originated in Canada during Precambrian time, 1 to 3 billion years ago. (Compare with no. 19, no. 23, no. 27, and no. 28.)