I am one of John and Jemima (Chisholm) Murchison's descendents and I have collated quite a lot of material on the family. I think I have the article you are looking for and am happy to send you a scanned electronic copy. I'd be happy to exchange any material and results of my family history researches if you should wish.
By the way, the Murchison rock is still in existence, located on a property somewhere in Kewanee, IL. I think Dave Clarke knows where it is (he is descended from Jemima's sister, Margaret Chisholm Johnson).