March 1948 The Queensland Naturalist 
93- 
the intervals of volcanic eruption ordinary terrestrial 
sediments (conglomerates, sandstones and shales) were 
deposited in the valley and the whole, pyroclasties and 
sediments formed the lower or andesitic phase of the Esk 
Series. Fossil plants, chiefly cyeadophytes. are found in 
some of its tuffs and shales. 
This dominantly andesitic phase was quickly followed 
by a traehytic phase with trachyte tuffs and lavas inter- 
calated with ordinary terrestrial sediments, forming the 
upper Esk Series. Kiel) fossil floras are foundjn sarmv-e^, 
the tuffs and shales, chiefly cyeadophytes. 
i wtfck 1 1 lie wit iT' ■ 
043 
t H l/'4i1<Tvoi nent 
,ved by that '^koi^ype 
io Bnndamba sT^WTmiy lOyyi^v 
11 , 1 / 1 <1 4 I + 
seed-bearing ferns, equisetalians and with 
their cuticles still preserved in a carboipivg^y.s-J " 
Then, still in Triassie times. an* 
occurred, which was followed 
mentation which formed the 
which spread far afield, even westwards inFo'Sfn^t'feat 
Artesian Basin, To-day, it is found in the Brisbane 
Valley only to the south of Esk. 
Possibly related to this earth movement, or perhaps 
later than it, there were injected into these Mesozoic sedi- 
ments of the Brisbane Valley particularly in the Mt. Bris- 
bane-Somerset Bam area, a series of magmas which 
because of their heat and their reactive emanations, 
caused the andesitic agglomerate at the dam site to be 
metamorphosed into the extremely tough porphyry which 
is so admirable a foundation for the wall. Where this 
metamorphism could not act. the andesitic agglomerates 
weather down very quickly into a friable mass. 
The petrology of the dam site is extremely interesting 
and lias been studied by C. W. Ball (1940. Proe. Koy. Soe. 
Queens. 52, p. 2). The first magma to rise up into the 
lower Esk Series was injected in two successive stages, 
and solidified as angite-diorite. Tt was followed by an 
acid magma which solidified into alaskite (nucrograph- 
icallv intergrown quartz and felspar) and gave off lac- 
coliths and dykes of quartz felsite. Late magmatic 
quartz-felspar-tourmalim veins are found cutting all of 
these. In the quarries and cuttings round the township 
of Somerset, most instructive studies can be made of tic 
metamorphism or metasomatism of the older rocks of 
each successive intrusive body. 
It ising through or in the Mesozoic rocks of the Valley 
