In 1831 when Smith was finally recognized by the Geological Society of London as the "fath

er of English geology," it was not only for his maps but also for something even more important. Ever since people had begun to catalog the strata in particular outcrops, there had been the hope that these could somehow be used to calculate geological time. But as more and more accumulations of strata were cataloged in more and more places, it became clear that the sequences of rocks sometimes differed from region to region and that no rock type was ever going to become a reliable time marker throughout the world. Even without the problem of regional differences, rocks present a difficulty as unique time markers. Quartz is quartz—a silicon ion surrounded by four oxygen ions—theres no difference at all between two-million-year-old Pleistocene quartz and Cambrian quartz created over 500 million years ago.

According to paragraph 4, it was difficult for Smith to distinguish rock strata because

A．the rocks from different strata closely resembled each other

B．he was often unable to find fossils in the younger rock strata

C．their similarity to each other made it difficult for him to distinguish one rock type from another

D．the type of rock between two consistent strata was always the same