In response to my comment concerning Fig. 3a and the appearance of structured residuals, the authors reply: “A concave-upwards trend means that the diamonds at greater distance from the surface are larger than for a constant vT. Larger means longer effective etch times and shorter access times, which in turn implies a greater average vT between their surface and the crack intersections, i.e., a vT increase towards the tip.”

I stand corrected; the quantities being graphed are unfamiliar and thus a little tricky to interpret, at least for me.  One insight that helped me is that the steady increase in steepness of the curves in Fig. 3a also provides a direct indication of the time lag in re-etching; if the etched track was infiltrated instantly, then the lines would be parallel.  In any event, the apparent non-linearity remains interesting, even if it implies a rising etch rate, and I look forward to the authors providing the raw data as part of their published manuscript. 

To the authors’ final comment (“We propose to refer to the reviewer's results (pers. com.?) and to moderate our conclusion.”), the Dpar measurements are in the supplement to Tamer and Ketcham 2020 (Tamer, M.T., Ketcham, R.A., 2020. Is low-temperature fission-track annealing in apatite a thermally controlled process?  Geochemistry, Geophysics, Geosystems, 21, e2019GC008877.  DOI: 10.1029/2019GC008877), and Tioga apatite fission-track ages and U determinations are reported by Roden and Miller (Roden, Mary K., and Donald S. Miller. "Apatite fission-track thermochronology of the Pennsylvania Appalachian Basin." Geomorphology 2.1-3 (1989): 39-51.).  If they want to also refer to a personal communication, these peer reviews have their own DOI, so that might be the modern way to do it.