Some googling tells me that the capping lavas (there are three flows in total) consist of "shoshonite, a potassium-rich basalt."
http://www.periclespress.com/geology_Table.html
Shoshonite is not a rock type I have encountered before. That's not unusual. There a host of variants of the fundamental rock types with local names. The potassium rich character would account for the colouration - orthoclase is a potassium feldspar. So your sample almost certainly is a basalt, but of an unusual composition.
I was interested to learn that the mountains overlook Golden, home to the Colorado School of Mines, which is world famous - at least in the Earth Science community. A dear colleague, who persists in buying up mineral rights in obscure places in hope of striking it rich, was educated there.
The shale like material you found between the flows is likely a fossil soil. The feldspar in the rock readily breaks down into clay minerals. I am slightly puzzled by the apparent freshness of the upper surface of the flow, but again working from a photograph is not the ideal way to go. :?
You can get a paper on "The Cretaceous-Tertiary boundary interval at south table mountain, near Golden, Colorado" here:
http://www.springerlink.com/content/d21th5921041342m/
It would be neat if that clay layer between the flows is the KT boundary itself. If you can just pop back up with your portable mass spectrometer and check the iridum content we would know for sure. :wink:
Alas, not. It seems, if this paper is correct, that the lavas are much younger.
http://www.buttecounty.net/dds/Plann...aFormation.pdf