Birch Mountains kimberlite field shales interbedded with kimberlite at some of the diatremes are consistent with a Late Cretaceous age (Late Albian to Maastrichtian). The volcanic rocks of the BM field consist primarily of crater-facies pyroclastic kimberlite and resedimented kimberlite. They include massive layers, as well as bedded and graded layers of coarse lapilli alternating with layers of finer tuff. Lapilli and olivine phenocrysts are set in a grey-green matrix of serpentine, carbonate minerals (calcite, dolomite and magnesite) and clay minerals. Phlogopite, oxide minerals (ilmenite, perovskite and spinel), apatite and pyrite are also present. Diamonds and microdiamonds have been recovered from samples taken from
Grammy Award for Best Rock Performance Recording Academy's wish to decrease the list of categories and awards and to eliminate the distinctions between solo and duo/groups performances. The Academy argued that any distinction between these performances is difficult to make, as "four-fifths of rock acts are groups, and even solo rock acts tend to be backed by a band". From 2014, this category has also included hard rock performances that were previously screened in the Best Hard Rock Performance and Best Hard Rock/Metal Performance categories, which are now defunct. Grammy Award for Best Rock Performance The Grammy Award for Best Rock Performance is an award presented