Jen swan

Hello

I found a large basalt rock in the river. It sturck me as strange. It took 17 hits with a concrete breaker to be able to knock a piece of the stone off, inside of the rock there was little pockets in the pockets there was a fine white powder . What could this powder be? There is no way the pockets could have been accessed from outside the rock. I was confused never seen or heard of this before. Can some one tell me what ut was and maybe how it came to be inside of a large basalt stone...?