eugeosyncline
noun
eu·geo·syn·cline (ˌ)yü-ˌjē-ō-ˈsin-ˌklīn 
: a narrow rapidly subsiding geosyncline usually with volcanic materials mingled with clastic sediments
eugeosynclinal
(ˌ)yü-ˌjē-ō-(ˌ)sin-ˈklī-nᵊl
adjective 
Word History
First Known Use
1942, in the meaning defined above