Thou who shalt stop, where Thames' translucent wave
Shines a broad Mirror thro' the shadowy Cave;
Where ling'ring drops from min'ral Roofs distill,
And pointed Crystals break the sparkling Rill,
Unpolish'd Gems no ray on Pride bestow,
And latent Metals innocently glow.
Approach! Great Nature studiously behold;
And eye the Mine without a wish for Gold.
Approach; but awful! Lo! th' Egerian Grot,
Where, nobly-pensive, St. John sate and thought;
Where British sighs from dying Wyndham stole,
And the bright flame was shot thro' Marchmont's Soul.
Let such, such only tread this sacred Floor,
Who dare to love their Country, and be poor.

Alexander Pope

http://www.poemhunter.com/poem/on-his-grotto-at-twickenham/