The following districts, however, where Devonian rocks prevail, have been the centres of gold mining enterprise:--Lucky Valley, Talgai, Gympie, Calliope, Boyne, Morinish, Rosewood, Mount Wyatt, Broken River, portion of Gilbert. In every case here cited, the country is traversed by trap rocks of a peculiar character, either diorite, diabase, or porphyrite; and tufaceous representatives of these are also found interstratified in the upper portion of the same formation, and occasionally throughout the other beds. At Gympie, the auriferous area is confined to veins traversing a crystalline diorite, or within a certain limit of its boundary, marked by the presence of fossiliferous diabase tufas. Whatever may have been the solvent and precipitant of the nobler metals in the auriferous veinstones associated with trap intrusions, all other but hydrothermal action may safely be eliminated, the very nature of the reefs, composed as they are of alternating layers of a promiscuous mixture of quartz, calcspar, pyrites, etc., affording unmistakable evidence on this point. The gold also contained in the trap dykes themselves is always accompanied by pyrites, both (according to Daintree), hydrothermal products separating out during the cooling down of the trap intrusions. Auriferous lodes, occurring in areas where hydrothermal action has attended trap disturbances of a special character in Queensland, are generally thin--to be estimated by inches rather than feet; but taken as a whole they are far richer in gold than those enclosed by sedimentary rocks. GRANITIC. Outcrops of granite extend along the eastern coast of Queensland from Broad Sound to Cape York, and inland as far as the heads of streams running direct from the inner coast range to the sea. Very little rock of this character is met with west and south of the Dividing Range which separates rivers flowing to the eastern and northern coast, and those trending south to the Murray or Cooper's Creek. The granites of Queensland vary very much in their crystalline texture, passing from true granites into porphyry and quartz porphyry. TRAPPEAN. Much stress has been laid on the value of certain intrusive trap rocks as specially influencing the production of auriferous veinstones in Queensland. The petrology of these may be divided into four type classes:--1. Pyritous porphyrites and porphyries. 2. Pyritous diroites and diabases. 3. Chrome iron serpentines. 4. Pyritous felsites. VOLCANIC.