To the left of where the plaque used to be, is this large dark rock body. This is a "magmatic enclave." It's dark partly because it's rich in biotite, partly because it's fairly fine-grained. One theory for enclave formation [geologists are not in agreement on this] is that enclaves represent blobs of a separate magma that ended up in the granite magma while both were still molten. Close examination will show that the enclave is elongate parallel to the magmatic foliation in the granite. Large K-feldspar crystals are found within the enclave similar to the ones in the granite.
These magmatic enclaves are scattered throughout the granite. Wandering around you are likely to see one somewhere.
Geo-nerd note: Some reference material says this is a xenolith, it's not, as shown by Smith, Diane and Wark, A. (1992) Magmatic enclaves in the Enchanted Rock Batholith, Llano Uplift, Texas. Geological Society of America Abstracts with Programs, 24,1,47.
