Hi!,
Galls are formed by bugs( although sometimes fungi are included in the definition). Cut one open. If you see a beetle, larvae or adelgid it is a gall. Is the tree a laurel(California Bay or Oregon-myrtle), waxmyrtle (pacific Bayberry), or other? It could be as simple as a willow--of numerous spp. I can't tell. Aromatic when crushed? I'm wondering if there is a berry beneath that stuff. If so, cut open the berry as well. Is it ornamental or natural? What is it's habitat and what does its fruit look like? Let me know what you find out!! interesting!!!