8 
MUSEUM BULLETIN NO. 4. 
Similar sections are to be seen in the South Branch Southfork 
river, within a half mile west of here, where the beds are repeated 
by folding. 
It has been noted that the Benton measures are virtually, 
wholly fine shales, indicating deep water conditions, while the 
Dakota beds are characteristic of more shallow zones of sedimen- 
tation. This relationship indicates a considerable and immediate 
subsidence after the volcanic activity in the region, and in fact 
even during the period of eruptions. The association of vol- 
canic action with a sinking terrane is mentioned by Geikie . 1 
Internal Stratigraphy and Structure . 
The volcanics as a whole are thoroughly stratified and, in 
nearly all cases, clearly water laid deposits. Thin beds are 
the rule, and it is rare that individual strata over ten feet thick 
are found. Irregular bedding is not uncommon. Layers of 
fine-grained, highly feldspathic tuffs are frequently seen near 
the base, and are scarcely to be distinguished from sandstones 
in the field. Some of these finer and more homogeneous tuffs 
are so dense and hard that they outwardly resemble lavas. 
Many varieties of tuff and agglomerate are present, from the 
types just mentioned, composed of angular fragments of feldspar 
in a finer ash matrix, to very coarse heterogeneous breccias of ex- 
plosive ejecta. Some beds are characterized by rectangular 
crystals of red and white or glassy feldspar up to an inch in size, 
embedded in a more or less homogeneous fine tufaceous matrix, 
and these rocks are easily confused with porphyries in the field. 
Again, distinctly water worn pebbles of varying sizes and propor- 
tions occur in a tuff matrix and these by increasing size of frag- 
ments pass into agglomerates. N o regularity in regard to sequence 
of strata has been observed in different repeated sections of 
the volcanics, and it is evident that the various beds are of a 
1 Geikie, Archibald. Volcanoes of Great Britain, vol. 2, p. 470. 
He concludes his remarks by saying,". there can be no question 
that with the one solitary exception of the Tertiary volcanoes, which were 
terrestrial and not submarine, all the British vents were carried down and 
eventually buried under aqueous sediments. Even the Tertiary lava fields 
have in many cases sunk down below sea level since their eruptions ceased.’ ' 
