Through this and other masonry the hot waters have been percolating for centuries, and have given rise to various zeolites- apophyllite and chabazite among others; also to calcareous spar, arragonite, and fluor spar, together with siliceous minerals, such as opal- all found in the inter-spaces of the bricks and mortar, or constituting part of their re-arranged materials.
"The Student's Elements of Geology"
Sir Charles Lyell