This was a special cake growing up. For us, it started with picking the blackberries so my grandmother could make the jam. My grandmother altered the recipie over the years, but I found this very old version in a letter written to my (then newlywed) mother. This would have been in the late 1950s. My grandmother wrote that she got the recipe from her grandmother, so that puts the recipe at about 100 years old. Jam cake just isn't the same without the crunch of the blackberry seeds. When I can't find jam with seeds, I add 1 tablespoon of poppyseeds for crunch.