Within a 13-mile radius of Kelly Butte there are over 32 volcanic vents;
within a 20-mile radius centered at Troutdale there are
90 volcanic centers. Most of these were originally
small cinder cones like Pilot Butte and Lava Butte near Bend, Oregon, but some
of them, such as Mount Sylvania in southwest Portland,
Highland Butte 10 miles southeast of Oregon City, and Larch
Mountain south of the Columbia River Gorge, were low,
broad lava domes of the type called "shield volcanoes".
The densest concentration of volcanic vents lies west of the town of Boring,
where 20 centers occur within an area of about 36
square miles. Because of this grouping near Boring,
Ray Treasher (1942) first gave the name "Boring lava" to the lava, cinders,
and ash which emanated from volcanic centers in the Portland area
within a time span of from perhaps 10 million to less than 1
million years ago.
-- Allen, 1975