In this stone masonry wall, some interesting looking stones have been used.
The stone in this photo looks as if it has been built like Swiss cheese around a lot of small pebbles.
Without having a mineralogist or geological background, I would guess this stone is one of two descriptions:
1. Igneous stone above the Earth’s surface (extrinsic) with vesicles and / or amygdales that over the millennia following formation filled with soluble stone deposits that moved through the stone.
2. Sedimentary or metamorphic stone that forms around the smaller stones.
Most stone forms slowly over time, neither possible explanation is a good explation for a passive creation of this stone, and that is why the stone is so interesting.