27a the actions of scattering or inserting things here and there(14) -I-E- - - - - S-O-S
23a a slaty diabase rock composed of fine volcanic fragments and dust(10) - - - - - S-E-N
Dauv
27a is INTERSPERSIONS (18d is DUNEDIN).
For 23a I have only ?C?A?S?E?N. Does anyone else have the 5 missing letters that I need for this puzzle?
britannica