Bob the Boulder is studying the local geology of MIT, and notices that as he walks in any direction away from campus, he encounters the same series of rock strata. Taking a sample of each, he determines that the one furthest from MIT has crystals containing the isotopes Uranium-238 and Lead-206 in equal proportions, while the rocks closest to MIT have the isotopes Uranium-235 and Lead-207 in equal proportions. Which of the following structures could MIT be sitting in the middle of?