Vesicular basalt, common in area's of volcanic activity & used frequently for railroad beds (but you already knew that). Anyhoo, found this particular piece this morning on my way in from work. It had some light colored shapes that caught my eye, so I picked it up. At first I thought recent biological in origin, but tapping them with another rock said otherwise. Some have ring bases that make them look kind of like hats. To give an idea of the size of these, the largest one in pics 3 & 4 is 2cm at the widest. Not sure exactly what they are, but I like them all the same. I can lightly scratch the largest one with a knife blade, so there's that. I'll try to remember to get some vinegar tomorrow to test for calcite, but I'm thinking they aren't that.

You might be right, although the serious lack of actual photos is a pain. And it explains why the vinegar I used may as well have been water for all the effect it had... other than making the thing smell like a pickle. Yet another form of silica. HHmmm... maybe they are eggs of a silica based life form.. 
The next pics are of a small area close to the spheres, but on another side of the broken vesicle/void. 1st one is with the dino-lites leds, 2nd, 3rd & 4th are with the leds shut off & using a hand held external led from 3 different angles. Creates completely different aspects of the same small area, but all 3 looks better than the original w/ straight on leds. 

The hair really helps keep it in perspective.
Thanks for the pictures.  The middle pic of the three doesn't even look like the same rock!

Are you seeing much green olivine or peridot in your basalt host rock?