Why might there be concrete inside my wall?
The early 1900's was the age of horsehair plaster. The plaster was installed in several coats over wooden laths. What you might be hitting is the plaster that as squeezed between the lath and landed on the soleplate. You should be able to tell if it is plaster by its' somewhat crumbly texture. Insert a long screwdriver in the hole and tap it with a hammer. If it is plaster it should easily break-up and leave you with clean hole.