Wikipedia has an interesting blurb about it: "Whilst he was in fact describing an actual geological feature - a laccolith which he saw as resembling a cactus 1 - he was also, tongue-in-cheek, commenting on what he saw as an absurd number of "-lith" words in the field of Geology.".    From Wordnik.com.  [Archive 2008-03-01] Reference

The laccolith (Greek laccos, cistern; lithos, stone) is a variety of intrusive masses in which molten rock has spread between the strata, and, lifting the strata above it to a dome - shaped form, has collected beneath them in a lens-shaped body with.    From Wordnik.com.  [The Elements of Geology] Reference

The large shear zones may converge and are the hosts for the mineralization, but finer grained porphyritic dikes and sills that are located near mineralization are emplaced as laccolith type fillings parallel to the shear zone with north south structural boundaries.    From Wordnik.com.  [The Earth Times Online Newspaper] Reference