Dendrobium: fast new growth but old canes wilting/ is it Nobile?

[Stickygreengo]

Hey Stefpix-
Pumice and "lava rock" are both igneous extrusive rocks that have similar characteristics (vesicles, etc) but are differentiated by their chemical composition. The lighter-colored rock is pumice, and is composed of quartz (silica). The "lava rock" is called scoria, and is composed primarily of iron, magnesium, and also silica (commonly what comes out of volcanoes forming on oceanic crust; think Hawai'i). Anyway, I don't know much about orchids yet, but my geologic knowledge tells me that the rock composition is not as important as rock texture as far as mediums go...