Looks like a layer of gypsum board and a layer of newer sheet rock.

Often, remodelers find it easier to simply install new material over old instead of removing the old and replacing with new. In this case, it reduced the usable space in the room by 1/2" on each wall---and was much less expensive to do.

I
had always assumed the two strata were different layers on the same board; on
the few occasions when I looked at a loose piece (i.e., cutting out a rectangle in order to install a new electric receptacle) both strata came out together and were bound together.
I am almost certain they are part of the same board. My parents had owned the house for the past 50 years and I'm pretty sure that no new layer was added to the wall during the time they owned it.

How difficult is it to cut? Sheetrock is very easy to cut and crumbles to dust, plaster is much harder to cut and breaks into chunks possibly with animal hair in the matrix. How will knowing the material help you? Are you planning to do some work with the walls? That might help us give better info.

I'm not planning to do the work myself, but will
be writing a request for bids on repairing cracks and then painting. I’d like
to be able to tell them precisely what it is they would be working on. Also,
previous attempts to repair cracks (not along seams) have not lasted long; perhaps
because the contractor had not properly identified the composition of the wall.

I posted this to one other forum as well. A couple of people there have speculated it is rocklath and fiberboard, particularly homasote. No one seems to have any idea how to repair cracks that have developed over time [notably when a neighbor's oak tree fell on the house]. To answer Raye Smith's earlier question, I'm pretty sure that at least one of the two layers is more difficult to cut than modern-day drywall.

Jean, I mentioned the receptacles because the piece cut out of the wall gave me a change to examine the wall board. As best as I can tell it is a "sandwich" apparently composed of rocklath and fiberboard. Interestingly, the fiberboard side faces the room.

neither of them look fibrous enough to be fiberboard. You can pick fiberboard apart with your fingernails.

Casey

Homasote is gray and looks like compressed newspaper fiber, which is basically what it is. It's not as hard as wood and you can definitely sink your fingernail into it.

I have a house with lots of Homasote. It can be also be tan and look like a paper bag.. Our other home built in the early sixties had a thick coating of plaster over the drywall made at that time. We went through a ton of cutting blades during a remodel, but it was solid and pretty darn soundproof.

"I have a house with lots of Homasote. It can be also be tan and look like a paper bag."

Then it is not Homasote as it has always been the same. Gloria you may have building board, or sometimes called beaver board..............

Well, I have a more or less definitive answer from two pros who came by to give an estimate and saw it up close. It is, as Sombreuil suggested, rock lath, and plaster [brown coat and then finish/white coat]. Don't feel badly if you didn't guess it; I even sent the photo to Homasote and they said it was probably Homasote! Thanks for all those who tried to help.

Rocklathe and plaster is not too bad to work with, as long as you remember how hard the final coat is, and that it will often fracture if you drive something through it.

I have found that you need to predrill holes for even the most simple work on the wall. Driving a small nail to hang a picture might lead to a half-dollar sized divot if you do not pre-drill.

Bruce