444 JENKINS— GEOLOGY OF THE [May 29, 



brown chalcedony. This may indicate that the schists were derived 

 from old sedimentary rocks. 



Granites. — The larger dikes which cut through the schists are 

 usually granites. These rocks are medium-grained with a pinkish- 

 gray color due to pink color of the feldspars. In one specimen the 

 following minerals are in the slides : quartz, plagioclase, and ortho- 

 clase in great abundance ; hornblende and magnetite prominent ; 

 titanite, garnet, and pyrite in smaller amounts. Other specimens 

 from one of the larger dikes show a quantity of microcline and in 

 addition to the other minerals zircon and apatite. In a specimen 

 which came from a decomposed portion of the granites, sericite is 

 prominent. The feldspars decompose leaving the mica flakes and the 

 quartz grains prominent at the surface. 



Log of Well at Baixa Verde, Rio Grande do Norte. 



The following is a log of a well with a six and three-fourths 

 inch bore taken at Baixa Verde, Rio Grande do Norte, at kilometer 

 84, elevation 162 meters. 



At — meters. 



Gneiss 5 



Dark granitic schist 10 



Granitic rock, hard 15 



Gneissoid granite 18 



Schist 24 



Fresh, hard granite, pinkish 39 



Gneissoid granite, with dark streaks 43 



Hard, gneissoid granite, pinkish 45 



Greatest depth reached 46 



From the above well, bored in crystalline rocks, it is clear that 

 the gneisses, schists, and granites are intermixed and intergrade. 

 This is typical of the way in which they occur in this country, — 

 first one and then the other, with perfect gradation. 



The Limestones. 



General Description. — The limestones vary in color from light 

 yellow to pure white. Some of the beds are bluish in appearance 



