Sime patterned art

So, the artist S. H. Sime, best known as the illustrator of Lord Dunsany, is best known for his black-and-white work. I knew from various researches during my dissertation he did some color work, but it was not until I had a chance to visit the Sime Gallery in Worplesdon that I realized how much color work he did, mostly landscapes but also a few portraits (including a memorable one of John on Patmos's vision). Some of these used a very unusual and striking technique I've never seen elsewhere. Of the two piece I hope will show up as links within this post, the first is called "Patterned Hills" and the other  I think "Dark Forest". Has anyone come across this technique before?

--John R.