The shapes in these two photos do resemble each other, but these two formations are very different both visually and geologically. While both are very dramatic, Devils Tower is a true "tower" of rock that rises alone above the surrounding landscape. El Capitan on the other hand is a protruding formation in the massive north wall of the Yosemite valley. Geologically, they are both of igneous origin, but they followed very different paths. Although they are both igneous, the current formations are not really volcanic, but rather are "intrusions" of molten rock that have been exposed by either being uplifted in the case of Yosemite and eroded in the case of Devils Tower. The rock itself is also quite different as Yosemite is granitic as a result of repeated cycles of glaciation while the Tower is still raw igneous rock.
I would refer you to the following links for more detailed info.
Yosemite geology
http://en.wikipedia.org/wiki/Geology_of_the_Yosemite_areaDevils Tower geology
http://www.nps.gov/archive/deto/geology.htmGeo
Thank you for the "rock" lesson, I find it very interesting, I'm always reminded of the addage the more you know...the more you don't know.
