[doodad]

Obviously they are weathering differentially as the spheres are being left as the matrix breaks down. I also noticed that the spheres are in most cases larger than the layer they occupy, which would also indicate that they "grew." The only other thing I can think of would be infilling of gas bubbles by a solution. However, they mentioned that large amounts of olivine was present which does not exist with quartz, so they are not siliceous unless a long time gap occured and a change in source materials provided the silica.

[doodad]

I just noticed also that the spheres appear to have formed on the top of the layers. I don't see any that are wrapped or crossed by the layering.