Calcified filaments of Ostreobium are common in beachrock on Grand Cayman Island. The alga exhibits an epilithic, chasmolithic, and endolithic mode of life. In its endolithic mode it bores into carbonate allochems and apparently uses the Mg and Ca so derived in calcifying the filament in its epilithic or chasmolithic mode. Glass fragments, cemented in the beachrock, are also bored by algae in a manner similar to the carbonate allochems. The Si dissolved by the alga in its boring phase is incorporated, along with Ca and Cl, in the algal sheath.The fact that algae can bore into glass suggests that they do not assume an endolithic life mode in order to obtain organic nutrients or moisture. The algae assume this life habit because of external pressures such as wave energy or simply because the algae bore into any available substrate. This example also demonstrates that algae are capable of secreting substances that can dissolve silica.