What carries the idea of rocks as artwork? ‘Massive weight,’ replies Michael Heizer in a gallery statement announcing his current exhibition of new stone and steel sculpture at Chelsea’s Gagosian Gallery. Sheer size, heaviness, and a certain kind of audacity in relocating huge pieces of nature blasted out of place and trucked into art-related settings are the hallmark of Heizer’s practice. In the recent sculpture, rocks seem to almost perch on thick planes of rusted steel in geometric shapes, setting up a dynamic interplay between manmade and natural forms that suggests both symbiosis and antagonistic struggle. (On view on 21st Street through April 16th).