Twenty-five new chemical analyses show Santiaguito's eruptive products to be soda-rich dacite of the calc-alkaline suite. The dome lava has differentiated quite significantly from the overwhelmingly abundant pyroxene andesite magma which makes up Santa María and the older volcanic rocks in the area. The rocks of the dome complex are uniform, gray-brown phorphyritic dacite and andesite, usually with oxyhornblende phenocrysts as well as plagioclase. Dome units are generally more grayish in color than the flows, are less vesicular, have more inclusions, and show little or no evidence of flow in response to gravity after extrusion. They comprise a much larger volume than the flow units. Spines and slabs stud the summits. The largest spine now preserved is on the La Mitad Dome; it is 200 m long and 70 m high. The shape of the dome units is sometimes circular, as in the case of the La Mitad and the El Brujo units, suggesting a simple extrusive vent. The Caliente unit was extruded from two or more vents and the El Monje dome can be subdivided into two elongate units on the basis of aerial photography control at various dates. Extrusive vents for the El Monje domes were apparently along fissures striking eastward.