Soon after the youngest layers of Siyeh limestone had accumulated on the
floor of the sea and while they were still under water, a mass of molten
rock was squeezed up from far below and extruded in the form of a
submarine lava flow over the recently accumulated sediments. Several
times this lava poured out forming a total thickness varying between 50
and 275 feet. One of the best exposures is on the west side of
Swiftcurrent Pass and in Granite Park just west and northwest of the
chalet. In fact it is this lava flow which gives the name, albeit
wrongly, to Granite Park. The material of the flow is very fine-grained
and dark (basic), in contrast to the light color and coarse grain of
granite. Nonetheless, many prospectors are wont to call every igneous
rock, regardless of its composition, a granite. A number of ellipsoidal
structures ("pillows") up to two feet in diameter within this lava
indicate that it was extruded under water. The Purcell is thickest in
the vicinity of Boulder Pass, where the trail traverses its ropy and
stringy surface for a distance of several hundred yards.
Later, after the Shepard and part of the Kintla formation were laid down
on top of the Purcell, another similar flow spread over the sea floor.