“ The general aspect of these hills is a succession of sharp peaks, formed by the 
Upheaval of portions of strata, with a change of angle, one side of each hill being a 
nearly even slope, with a steep escarpment on the opposite side. The steep side, 
however, shows a columnar structure, which is vertical, and not a right angle, with the 
slope on the opposite side of the hills. Some of these peaks, as Mount Barney, 
Wilson’s Peak, Mount Lindsay, and Mount Mitchell, have an altitude of 3,000 and 4,000 
feet, and show the porphyry to exceed 2,000 feet, but in every case resting on car- 
bonaceous rocks [Ipswich Formation]. Though the porphyry sometimes flowed over 
considerable areas in thin sheets, and shows a cellular structure, yet the greater part 
must have been less fluid when erupted than the basalts. 
“ Besides the principal mass betw'een Cunnmgham’s Gap and Mount Lindsay, 
there are several detached points of eruption, as Knapp’s Peak, Mount French, Mount 
Edwards, Flinders Peak, and Mount Goolman, all situate to the south of Ipswich, 
Mount Esk, on the Upper Brisbane Eiver, Kangaroo Point, Bowen Terrace, and 
Spring Hill, in the town of Brisbane ; also the Glasshouse Mountains. 
“ In the town of Brisbane the porphyry is erupted through the Devonian slfite, 
fragments of which are disseminated through the porphyry, while portions of silicifled 
wood are embedded in the lower surface of the erupted mass.” 
In another place it will be seen that the Kangaroo Point and Bowen Terrace rock 
is regarded as an altered brecciform ash rather than as a true crystalline rock. The 
Glasshouse Mountains, also, were regarded by Stutchbury as altered sandstone. 
The granite of the Pentland Hills is metamorphic, “ alternate layers of coarser 
and liner material betraying its originally sedimentary origin,” as has been observed by 
Daintreo, and subsequently by myself. 
As many of the most important metalliferous fields of the Colony occur in the 
group of rocks now referred to, lithological observations will be found in the descriptions 
of the localities. The reader is also referred to Mr. A. W. Clarke’s Micro-Petrographical 
^^otes printed with this work. 
MINES CONNECTED WITH GEANITIC, &c., EOCKS. 
HEEBERTON TIN EIELD. 
This field, which is one of the most important sonrees of lode tin in Australia, was 
only discovered in 1879. 
The “country-rock” of this field is divisible into three classes: (1) a perfectly 
normal granite; (2) a porphyry of <piartz and felspar (quartz predominating), with 
nnca as an occasional or accidental and not essential constituent ; and (.3) highly inclined 
greywackes, quartzites, and shales — belonging, there is every reason to believe, to the 
Gympie Formation. The rocks of the first class, which extend northwai’d from Watson- 
ville, appear to be nearly barren of tin ore, at least in the neighbourhood of llerberton 
and Watsonville, although at Ecturn Creek they contain tin lodes. Those of the second 
Were for a time regarded as the only seat of the tin depo.sits ; but it is now questionable 
■whether those of the third class do not excel them in this respect. 
The porphyry rocks arc intersected in every direction by large “ elvan ” dykes 
(compact, highly silieated, yellowish or greenish felspar base, with blebs of quartz).* 
icse elvans contain a good deal of arsenical pyrites, but — except in the case of the 
iree Star Mine — have not yet j^roved to be tin-bearing to any great extent. Dykes of 
quartzose chlorite and quartzose serpentine — probably originally intruded among the 
porphyry as quartz-diorites, or as rocks more or less of basaltic type, and subsequently 
Sec Mr. Clarke’s Micro-Vetrographical Notes. 
