Authigenic laumontite is extensively developed in sandstone beds of Eocene to Miocene age exposed in the western Olympic Peninsula, Washington. In these rocks, laumontite primarily replaces calcic plagioclase, but it also forms cement and fills veins. Degree of alteration varies locally, depending primarily on the presence of calcareous cements, but also on porosity and grain size. Metamorphic grade increases eastward toward the core of the Olympic uplift and passes, apparently gradationally, into the prehnite-pumpellyite facies. Similar alteration reactions should be anticipated in other deeply buried clastic sequences in the Oregon and Washington Coast Ranges.

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