CONSANGUINITY OF ERUPTIVE ROCKS. 599 



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 pos- 

 sible 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 Pocos 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 hun- 

 dred miles away than at those points themselves. Instead, 

 therefore, of returning to Cabo Frio an excursion was made to 

 Pocos 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 z are seen included in 

 foyaite and vice versa masses of foyaite are included in phono- 

 lite. Considerable masses of a leucite rock, the first known from 

 South America, cut by and buried under phonolite and present- 

 ing 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 know- 

 ing their association, would ever think of calling them anything else, although some, with 

 that knowledge, prefer to call them nepheline-syenite porphyries or tinguaites. 



