Geology of the St. Croix Dalles. — Berkey. 145 
ture seems to be due to flowage. The microscope adds 
nothing to the distinctness of these bands. They are the only 
traces of flowage structure to be found in this rock with the 
exception of that clearly shown in certain grains of the vol- 
canic tufif. 
The ophitic texture so abundantly developed, the mineral 
constitution of these rocks and their holocrystalline condition 
are characters belonging to a "diabase." 
The porphyritic, ophitic and other structures developed 
in the rock necessitate some qualifying terms. But the fact 
that all these varieties are only local phases of one parent rock; 
whose definition may well be broad enough for uny or all of 
them, leads to the conclusion that this igneous rock belonging 
to the St. Croix Dalles area is most proper I)- designated a 
"diabase." The terms "porphyritic diabase," "ophitic dia- 
base" and "amygdaloidal diabase" are explanatory of its local 
variations. 
Volcanic Breccias. In the immediate vicinit\- of the upper 
Dalles on both sides of the St. Croix river loose pieces of 
breccia were found. -Subsequently this breccia was found in 
place just above the public school building in Taylor's Falls. 
The fragmental nature of the rock is not readily apparent, for 
the rock in place is as hard and compact as other portions 
of the outcrop. But on closer inspection it is seen that the 
fragments of diabase are angular, irregular and of all sizes, 
and lie imbedded in a matrix of finely crystalline secondary 
minerals, chiefly epidote and quartz. It occurs in one of the 
division zones between two flows. The occurrence of a 
breccia was not noted until after a division plane between 
two flows had been determined upon at this point entirely 
upon other characters. Later the discovery of an ash at the 
points previously determined upon as divisions between flows 
came as a very welcome proof of the accuracy of other ob- 
servations. 
A breccia results at the contact of two lava flows when- 
ever the earlier one presents a surface sufficiently craggy and 
vescicular for crushing into a broken mass; or whenever the 
later flow supports at its front a particularly abundant crop of 
cooled surface-cakes and cindery debris, which are continu- 
allv rolled beneath the advancing stream. 
