“There is always some trace, Roger tells me, of the history of things, the impressions humans have left on them. The term for the study of rock layers in geology is stratigraphy, but it is also used in archaeology to describe the technique of seeking out the contexts of rocks, discovering the events that have left detectable traces on their surfaces – in the same way we might scan one another’s bodies, looking for those distinctive lines and marks which tell us something unspoken about the stranger opposite us on the train, or the friend we grew up with but have not seen for years, or the person we are falling in love with. A geologist’s task is to see beyond the ways in which time tries to smooth out difference, examining layers in order to isolate each shift to our world, to feel every fault line. We discuss how hard this is to do this with people, to imagine our lives not as one continuous line, but comprised of hundreds of versions stacked up behind us, and hundreds more ahead of us too, like those pairs of facing mirrors that make your reflection curl up infinitely on either side of you.”