The mica is deformed by the heated metal cone on top of it and a bulge is formed as a result of the heating which is what makes the cone roll. The copper sheet under the mica cools the heated place quickly; thus the cone is in contact with cool mica at every moment and as this cool place is heated it too is deformed. If a sheet of glass lay under the mica, there would be no such cooling of the mica, which would soon grow hot and the cone's movement across it would stop.