J. G. Branner — Geology of Fernando de Noronha. 157 



make up the body of the hill. The limburgite (No. 14) of the 

 west base of Atalaia Grande forms a dyke either in, or west of 

 and adjacent to, the amphibole trachyte (No. 10) mentioned 

 elsewhere. Inasmuch as the eastern side of Atalaia Grande is 

 exposed in one place down to the water's edge and is seen to 

 be composed entirely of phonolite, these dykes of trachyte and 

 limburgite are probably exterior to the body of the hill. Lim- 

 burgite (No. 65) was found also in the peak of volcanic tuff 

 which rises upon the narrow neck connecting the Sapato with 

 the island. It there occurs associated with travertine (No. 66) 

 and volcanic bombs. (Nos. 3 and 58). The two small isolated 

 rocks known as the " Dois Irmaos " are not accessible, but as 

 seen from the main island they have the appearance of being 

 composed entirely of rudely columnar basalt. 



Volcanic bombs (No. 48) occur in situ on the northern 

 and near the summit of the Morro Francez. The east side of 

 this hill, where it slopes down to the sea, is much checkered 

 by dykes varying in thickness from two feet to eight, which 

 cross each other at all angles. The bench of hard rock which 

 skirts the base of the hill, and which is uncovered at low tide, 

 varies in width from zero to three hundred and fifty feet. On 

 its outer margin it is bordered by calcareous formations. On 

 this bench the dykes are beautifully displayed at low tide. 

 Immediately east of Capim Azul, a cliff more than 300 feet 

 high is composed almost entirely of volcanic bombs and tuffs 

 in beds dipping to the south at a high angle, and capped by 

 jointed basalt. In size these bombs vary from that of a pin's 

 head to the size of a bushel. Volcanic bombs occur also in the 

 tuffs about the western end of the island, but they are nowhere 

 so abundant or of such size as at the cliffs of Capim Azul. 



Tuffs. — Tuffs occur about the northern and eastern sides of 

 the Morro Francez, but are especially abundant about the 

 western end of the island, where some of the beds are more 

 than one hundred and fifty feet thick. 



From the east side of the Morro Francez near the Pontinha 

 are heavy beds of loosely consolidated tuff (No. 118), consist- 

 ing of a mixture of angular fragments of rocks of many kinds 

 which vary in size up to that of a millstone and larger ; the 

 beds slope down to the immediate beach of solid rock. This 

 loose material is unlike the basaltic tuff of the western end of 

 the island ; it is greenish gray and without any appearance of 

 stratification, while that of the Sapato and Capim Azul is more 

 or less stratified and brownish. 



The cliffs about Barro Yermelho and for some distance 

 along the south side of the island, extending from the bed of 

 volcanic bombs at Capim Azul to or near the Portao, are of 

 some form of tuff (No. 62). The rock is soft and reddish, and 



