Derby.] -l-bo [Feb. 21, 



betas. The lower part of this hill is composed of felsite, above which the 

 Upper Silurian beds form a magnificent overhanging cliff. In the lower 

 part of the second cachoeira, called Vlra-Mundo, the Silurian beds rest on 

 syenite. The dip is approximately 5° S. S. W., the strike being X. 65° W. 

 I estimate the total thickness of the series at about 1,000 feet. 



Tlic character of the beds is remarkably uniform. They consist almost 

 exclusively of hard argillaceous and micaceous sandstones, generally thin- 

 bedded, but with some massive beds of pure sandstone. The color is very 

 variable, being white, yellow, red or purplish, but the predominant color is 

 some shade of red, generally mottled or banded. Limestones are entirely 

 lacking, and schists are rare and of slight importance, as regards their 

 thickness, but interesting on account of their peculiar characters. One set 

 of beds of cherty schist, about 20 feet thick, is found at the base of the series, 

 in contact with the syenite. This rock looks like one that had suffered some 

 alteration, and this appearance might be taken to prove that the syenite is 

 of igneous origin, and that it had been ejected after the deposition of these 

 beds, effecting an alteration in them. As, however, the altered appearance 

 is less marked in the part of the schists which is in immediate contact with 

 the syenite, than in the upper portion of the bed, I believe that their peculiar 

 appearance is due to some other cause. Another schist of undetermined 

 thickness occurs at the base of the cliff, forming the front of the Oiteiro do 

 Cachorro. It consists of a soft clay, impregnated with alum, which also 

 occurs abundantly in free crystals. 



At the foot of the Cachoeira Vira-Mundo, and just above the cherty rocks 

 above mentioned, there is a bed of fine-grained, yellowish sandstone, con- 

 taining a few fossils of which we collected with considerable difficulty suf- 

 ficient to determine the age of the formation. The fossils are all in the 

 state of casts and, except a species of Beyrichia and a fragment of a Trilo- 

 bite, are all Molluscan. The most common is an Orthoceras, which is how- 

 ever indeterminable. The genera, Rhynchonella, Orthis, Chonetes, Stropho- 

 (lonta, Lingula, Pholidops, Bucania, Conularia and Ctenodonta are repre- 

 sented. Among these the species Orthis liybrida Sow., Lingula cuneata 

 Conrad, and Bucania trilohata Conrad are recognizable. In the Oiteiro do 

 Cachorro are thin beds of shaly sandstone, with well marked fucoids, ap- 

 parently of the species Orthrophychw Harlani Conrad. These fossils 

 indicate a close correspondence with the Medina sandstone of the Niagara 

 group. Throughout the whole series worm -tubes are abundant. 



The same series of beds were met with on the Curua and Maecuru, with 

 characters identical with those just described for the Trombetas. On these 

 rivers the Silurian rocks form cachoeiras, that were impassible with the 

 means at our disposal, and for this reason we did not succeed in reaching 

 the base of the series, where the fossil iferous beds occur. Worm tubes and 

 indeterminable fucoids were, however, met with. The Upper Silurian has 

 not yet been recognized on the southern side of the valley, but, as all the 

 sections on that side are very incomplete, it is by no means certain that 

 they do not exist. It is possible that the cherty beds of the Tocantins, 



