In the area between the villages of Siquirres and Turrialba along the Rio Reventazon a number of teschenite sills outcrop which are associated with lavas of alkali basalt and basanite. The sills are up to 40 m thick and are emplaced in Tertiary marine sediments. Some sills are homogeneous and comprise olivine, salite, a brown amphibole, plagioclase (An55), analcime (30-35%) and a little biotite, apatite, zeolite, opaques and sometimes hauyne. Other sills pass from a mesocratic facies at lower levels, rich in olivine, clinopyroxene and labradorite with interstitial analcime and a little biotite, opaques, apatite and zeolite, to leucocratic rocks at higher levels containing only a little salite, andesine, much more abundant interstitial analcime, alkali feldspar, biotite, opaques, zeolites and apatite. There is some acicular aegirine-augite or aegirine and in some sills nepheline occurs.