The mountain building process that created the Appalachian Mountains is the source of the shearing that created the alpine-type fissures we find throughout the entire chain.  In this short animation I created, you can observe how randomly placed structures (stones in this example) seem to create linear patterns or weaknesses between them and perpendicular to the foliation as it becomes sheared.  These perpendicular weaknesses are where I believe fissures may form.  Understanding that at the time of this shearing, tremendous pressure and heat have made all structures malleable and perhaps partially melted, thus flattening, stretching, and "pancaking" all the structures into linear layers.  The fissures would provide the necessary space for the recrystallization of the quartz and other rare minerals to grow.