“In these, the spicular aspect grades abruptly inward into dense cryptocrystalline silica, which forms the bulk of the specimen.”

“They are largely silicified replacements of the once opaline skeletons, but details of their spicular and skeletal relationships are moderately well preserved.”

“Subsequent microscopic study of the outer layer shows that, in a few specimens, patches of the microcrystalline material grade laterally into vestigial spicular fabric.”