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CONSANGUINITY OF ERUPTIVE ROCKS.
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by numerous dykes of phonolite, monchiquite and augite-syenite porphyry, as well as of diabase which, as it occurs everywhere in the gneiss regions of Brazil, was not taken into account. Although nothing definite on the field relations of these various rocks could be made out, the idea suggested at Tingua of a possible genetic relation between foyaite, phonolite and monchiquite was strengthened by this repetition of the association and mode of occurrence, that is to say, of a central mass of foyaite with apophyses of phonolite and monchiquite. Aside from this, the association of foyaite with augite-syenite, with a plagioclase rock and with tuff of a volcanic character, suggested other lines of investigation not in accord with the usually received notions regarding these rocks.

Before a second projected excursion to Cabo Frio could be realized a chance specimen of foyaite from the Poços de Caldas in southern Minas appeared at the Rio Museum. As a railroad was under construction in this region the idea at once presented itself that, aside from a study of this district, possibly Tingua and Cabo Frio might be studied more advantageously several hundred miles away than at those points themselves. Instead, therefore, of returning to Cabo Frio an excursion was made to Poços de Caldas where the expectations formed were more than realized. About twelve kilometers of almost continuous rock cutting up a steep mountain slope giving one of the finest and most varied exposures of eruptive rocks in the world, was found. Here immense masses of tuff are seen to be cut by both foyaite and phonolite; dykes and sheets of foyaite pass into phonolite at their margins; small masses of phonolite[1] are seen included in foyaite and vice versa masses of foyaite are included in phonolite. Considerable masses of a leucite rock, the first known from South America, cut by and buried under phonolite and presenting tuffaceous facies also occur. Small stringers of augite-syenite were noted in the tuffs and phonolite, and nests of

  1. ↑ The name phonolite is retained for these rocks since no petrographer, not knowing their association, would ever think of calling them anything else, although some, with that knowledge, prefer to call them nepheline-syenite porphyries or tinguaites.
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