superstratum

After a time the superstratum of rock, which is full of cracks and seams, is undermined and precipitated into the chasm below.

The superstratum was very light, and brownish black, the remainder yellowish brown, the yellow tints as well as the stiffness increasing downwards. 

And before she could stop him, he had pounced upon it and pulled it out, upsetting a superstratum of gowns in the process.

After leaving the narrow valley which the river has cut for itself through a superstratum of yellowish clay, the country becomes nearly level--a dreary plain, covered with fern and the manuka bush.

In what he has to say about the Indians, a subject that lies as a superstratum under his work, he is anxious to hear all that can be said.