Tabberabbera Shales and Cobannah Quartzites. 
■17 
Tiie Tabberabbera Shales and Cobannah Quartzites. 
Another group of rocks of Middle Devonian age was found by 
Mr. A. W. Howitt to occur at Tabberabbera, at the junction of the 
Mitchell and Wentworth Rivers, where, in black shales, associated 
with limestoues,he found the characteristic Spirifcra tavicosfata of 
the Buchan and Hindi limestones. Besides the black shale and 
limestones, there is a series of slates, flinty shales, sandstones, and 
quartzites, apparently belonging to the same group, extending 
south-westward from Tabberabbera beyond Cobannah, and over to 
Maximilian Creek, a branch of the Freestone. Creek, running into 
the Avon River. 
These rocks have been folded, compressed, indurated, and meta- 
morphosed, and so nearly resemble somewhat altered Silurian 
strata that, prior to Mr. Howitt’s investigations, t hoy were classed 
as such. They are, however, according to notes furnished me by 
Mr. Ilowitt, nnconformable, and their folding has been effected 
subsequently to that of the Silurian rocks. On the north and east 
these rocks appear to be bounded by granite and Silurian rocks, 
while on the south and west they pass under the Upper Devonian 
rocks of the Iguana Creek and Maximilian Creek. 
With the Middle Devonian rocks we lose the more conspicuous 
indications of metaraorphisra, contortion, and folding which form 
such prominent characteristics of the Lower Palaeozoic and of the 
last described group of the Devonian rocks. 
The rock-bands in the newer formations, including that to bo 
described next in order as Upper Palaeozoic, though generally up- 
tilted and undulating, are in some cases horizontal, and have 
nowhere been folded or metamorphosed to the exteut observable 
in the older rocks. 
The Tabberabbera and Cobannah rocks may with tolerable 
safety be regarded as Middle Devonian ; they are certainly not of 
more recent date, and, as above stated, show an amount of 
undulation in their beds equal to that observed in the Silurian 
rocks, but are unconformable to the latter, while the Upper 
Devonian rocks of Iguana Creek, which immediately overlie 
them, rest almost horizontally on their upturned and denuded 
edges, showing that, notwithstanding the close geological relation 
between the Middle and Upper Devonian Groups, a very con- 
spicuous stratigraphical unconformity exists between the two. 
In fact, the observed stratigraphical relations to one another 
of the various groups of Paheozoie rocks naturally suggest their 
classification in three main divisions — (1) Lower Palaeozoic, 
including Lower and Upper Silurian ; (2) Middle Palaeozoic, 
comprising the Lower and Middle Devonian ; and (3) Upper 
Paheozoie, including the whole series of the Upper Devonian 
rocks and those provisionally classed under the general term of 
Upper Palaeozoic. 
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