. Evidence from moving clouds, summarized by de Vaucouleurs, supports the author's suggestion that exten- sive winds of monsoon type occur on Mars. The volcanic hypothesis, developed to explain the banded and oriented permanent maria and pointed "bays," accounts successfully for the greatest temporary changes observed on Mars. In several cases, a horn-shaped dark marking fanned out from a well-defined small source and showed curvature consistent with the Coriolis effect of the planet's rotation. Examples are Hydaspis Sinus in 1858, the distorting appendage of Solis Lacus in 1926, Phasis in 1877, and Cyclops ca. 1800. Variable canals, such as Thoth and Cerberus, can be interpreted as due to variation of activity in certain oases. Shifting boundaries of maria are interpreted as due to aeolian deposition of light-colored desert dust or of dark volcanic ash, accompanied by variable volcanic activity and changes of wind direction. In particular, the seasonal change of outline of Syrtis Major is explained by seasonal reversal of the winds. Distances of transport of ash on Mars are not excessive as compared with recorded cases on earth from Vesuvius, Mayon, etc. Amounts of ash required for any of the temporary markings are only a few cubic kilometers, fairly com- parable with ejecta of major terrestrial eruptions. Volcanism on Mars appears not to exceed greatly, if at all, that on earth at the present time.