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Why must geologists estimate some Silurian boundary ages instead of measuring them directly?

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Silurian paleogeography — Silurian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the middle part of the Silurian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

Geologists must estimate Silurian boundary ages through extrapolation rather than direct measurement primarily because of the scarcity and condition of suitable igneous rocks.1

To determine absolute dates, geochronologists rely on the geochemical analysis of radioisotopes found within igneous rocks. However, these rocks are rarely found in close proximity to the specific boundaries that define the Silurian Period. Furthermore, many igneous rocks that might otherwise be useful have been subjected to metamorphism over time; this process reheats the rock, which effectively resets the radioisotope clock and renders the original age data unreliable.1

Because of these limitations, geologists are forced to rely on a very small number of viable igneous localities. They establish the age of the Silurian Period by extrapolating from the limited data points available in the older Ordovician and younger Devonian rock layers.1