Geology has lots of words for things that aren't there- that is, holes. In this case, the holes represent gas coming out of solution in molten rock; you can think of it as effervescent lava. When the lava cooled and solidified, the bubbles remained. We call these vesicles, and the texture, vesicular. It's extremely common in lava flows. This was a loose cobble that had rolled down the slope, so I didn't see the actual exposure, but often, the vesicles are more abundant toward the top of the flow- just as with bubbles in water, they tend to float to the top (though lava is much more viscous than water, so the rate of rise is slower). The concentration of bubbles to the top of a flow can give an observer a good sense of what direction was up at the time. In this area, though, I have no reason to believe there has been significant folding or deformation to make the question of stratigraphic "up" an issue of concern.