The large number of glacial boulders in Houghton gives us lots of information about lava flows, even though outcrops are not very common.  In west Houghton two boulders give us pieces of rare parts of the lavas.  There is a breccia where cobbles and pebbles of amygdaloid are suspended within a massive grey fine-grained lava.  This rock may have formed when lava flowed over and engulfed a flow top, and the amygdaloidal pieces were entrained and carried away by the flow, ending up suspended within the flow interior.  This is unusual, but something expected.

A more unusual flow part is a second, smaller boulder where we find a pinkish, feldspar-rich igneous rock that likely represents a pegmatitic horizon that is an extremely low temperature crystallization of the flow interior.  This could be called an extreme doleritic segregation. These kinds of rocks might only be found in the largest lava flows, and only in the center of the flow, where solidification took the longest.  Thick and pink colored pegmatitic sections of large lava flows have been found in outcrop within the Greenstone and Copper City flows. The location of this boulder and the one with dolerites nearby are very close to the bedrock location of the Greenstone flow. So these boulders may not have been moved very far by the glacier. These vuggy and bubbly layers have large plagioclase and a variety of mafic minerals and may be related to granophyric rocks which are very rarely encountered. The many centuries required to solidify and lava flow hundreds of meters thick may give rise to an “evolved” liquid magma which is gassy and lower in temperature than basalt.  We are unsure whether tis pegmatitic explanation is correct, however.  The alternative is that we have yet another exotic alien, a piece of ancient Canada masquerading.