Though made more than a century ago, above comment by the "Father of Petrography" is still valid. Whereas we have made considerable progress in understanding shales and mudstones, these rocks are so much more diverse than other lithologies that much needs to be done before we can be confident to understand them.

From an intellectual perspective, Shale Research is clearly a Frontier of Sedimentary Geology and will contribute much to a better understanding of earth processes, earth history, and earth system interactions. Due to recent global developments it is also becoming crucial at another frontier - the exploration and utilization of vast, yet untapped, hydrocarbon deposits in oil and gas shales.   
Watch Mud and Shale TV - its fun ! 

We also are investigating mudstones on Mars through involvement in the Mars Science Lab Mission that arrived on Mars in August 2012. The mission is still young, and many exciting discoveries in sedimentary geology await us in the years to come.