The Treasure of Atlantis was first published in 1916, a lengthy part of "All Around" magazine's December issue. The same magazine also published works by Haggard and Burroughs. Dunn's yarn of Amazonian/Atlantean exploration is a fitting companion.
The plot - especially to those familiar with Haggard - is predictable.
Evidence of a lost (white) civilization in some remote (non-white) part of the world falls into the hands of an uber-masculine British lord/explorer and his quirky scientist companion. The two men - one bored, one insane - jump at the opportunity to prove some sort of madcap theory by exposing themselves to tremendous physical danger.
In this case, they're off to Brazil, to prove the existance of some sort of inexplicable Cretan land-bridge. The lord - Samuel Morse - and his companion - Gordlan Laidlaw - stomp around (very briefly) in the wilderness, and quickly uncover the lost city of Atlantis.
Like many of Haggard's adventures, Atlantis is a shining bastion of lost, classical culture - ruled by a physically beautiful noble class and the invariably-corrupt priesthood. The bathrooms have hot and cold running water and the priests have hot and cold human sacrifice.
The Treasure of Atlantis really is charmingly predictable in most cases. Morse is distressingly perfect and thoroughly uninteresting. The love interest - Leola - is also a paragon of 1916 pulp-feminine perfection ("pale-skinned virgin, likes long walks on beaches, moonlit nights and men that keep her in her place"). Profesor Laidlaw is no exception. Similar to Doyle's Professor Challenger, Laidlaw is an intimidating, physically powerful (but not handsome) man, who respects other manly-men and likes to yell at people. Laidlaw also, in a vain attempt to be more interesting, is oddly fussy about his eggs. This point is made quite frequently.
Dunn adds something slightly new to the genre with his intimation of the lost city's moral dissolution. Atlantis has been an isolated, decadent society for over a thousand years, and it shows. The gladiatorial games are just a little too brutal and the crowds are just a little too eager for blood, superstition and sacrifice. There are also several brief references to a declining population. Underneath its gilded exterior, Atlantis is rotten to the core. Unfortunately, Dunn only takes the time to hint at this corruption before returning to the swooning Leola and muscle-bound Morse.
Still, this is a fun (if uncomplicated) story. The good guys win, the bad guys lose, and there are lots of sword-fights, boxing matches, mad priests and exploding volcanoes to keep things exciting.