I'm studying minerals..I understand that if molten rock cools slowly undergound, then large mineral cystals start forming. AND if that rock cools quickly at the surface, small cystals form. My ? is ..sometimes rock has both large and small mineral crystals ..how can that be explained?

Question

I'm studying minerals..I understand that if molten rock cools slowly undergound, then large mineral cystals start forming. AND if that rock cools quickly at the surface, small cystals form.  My ? is ..sometimes rock has both large and small mineral crystals ..how can that be explained?
Lost...
thanks for responding..Kira G.

Ms. Sue · Answer

The rocks with both small and large crystals is probably a conglomerate rock. Check this site for information. (Broken Link Removed)