schist

liuske Any of a variety of coarse-grained crystalline metamorphic rocks with a foliated structure that allows easy division into slabs or slates.
1890, George Huntington Williams, The Greenstone Schist Areas of the Menominee and Marquette Regions of Michigan, US Geological Survey, Number 62, https://books.google.com.au/books?id=7dd-vcewuLwC&pg=PA74&dq=%22schist%22%7C%22schists%22&hl=en&sa=X&ved=0ahUKEwjdhPzKyYHQAhUILpQKHZrwAikQ6AEIKzACv=onepage&q=%22schist%22%7C%22schists%22&f=false page 74,

Although it shows under the microscope traces of the original gabbro structure and of the original gabbro minerals, no one would hesitate from a microscopical examination to class it with the schists.

Narrow bands of schist are interstratified with these limestones, and in turn narrow beds of limestone are interstratified with the schists.

Millions of years ago the mica schists surrounding the old Brandberg West Mine became folded and concertinaed by enormous horizontal pressures.