Pockets filled with moldable clay?

[ShamrocksSecret]

I've busted open 2 separate large rocks that have a small pocket full of moist clay. There aren't cracks leading to these pockets. I'm curious to know what this clay is and how old it could possibly be. Has anyone else seen this? Is it common?

 

[Red James Cash]

Those are oossibly gas pockets from when the rock was formed.Crystals form in gas pockets.