Derby.] lb4: [Feb. 21, 



stones, have a strong easterly dip. As I remember it, the deposit appears 

 to be superficial, and I doubt if it is of economic value. 



"Opposite the Praia Grande is a verj long narrow line of rocks, running 

 south a few degrees east, and flanked by the schistose rocks, which here 

 present the ordinary eastward dip. The line of rocks is formed by a nar- 

 row outcrop of diorite, which I suspect to form a dyke. This diorite is 

 much cracked and, decomposing concentrically, the fragments give rise to 

 a confusion of rounded blocks. 



"Nearby, the slaty rocks again appear, with the cherty rocks appa- 

 rently overlying them in discordance of stratification. These latter rocks 

 may therefore be of much later origin. In one place I thought that I ob- 

 served signs of horizontal stratification. Near the upper end of an enor- 

 mous sand bank, called Praia Grande, the slaty rocks crop out again, the 

 strike being N. 30° W. and the dip 2V E. 



"The Cachoeira de Guariba is formed by the outcrop of a series of meta- 

 morphic rocks, an alternation of shales, quartzites and limestones, extend- 

 ing across the river, forming a sort of dam. The strike here is somewhat 

 irregular, but usually a few degrees west of north, the dip being eastward 

 and at a moderate angle. I could not ascend above the Cachoeira de Guari- 

 ba, from lack of time and of a proper boat. From all that I was able to 

 judge, the metamorphic rocks must extend much further up the river, and 

 it would be very important to have them examined. Whether the whole 

 series that I saw belongs to the same geological horizon or not, I was un- 

 able, in the absence of fossils, to determine, but, after my studies of the 

 Carboniferous and Devonian of the Amazouas, I think there can be little 

 doubt that the series is Silurian. 



"It is interesting to note the dip of these rocks, which is pretty con- 

 stantly towards the east, the strike being remarkably northerly. The fact 

 of the occurrence of trap dykes is also important. I saw no porphyries like 

 those of the lower falls of the Tapajos, and I cannot help thinking that 

 the Tocantins beds above described are newer than those of the Tapajos." 



The metamorphic rocks of the rapids of the Tapajos were described by 

 Prof. Hartt in the Bulletin of the Cornell University. They consist of 

 quartzites and other rocks similar to quartzites, but without apparent 

 granulation, the beds being tiaversed by enormous dykes of porphyry and 

 diorite. They are very compact, of a red or chocolate color, frequently 

 marked by little green points, due to some undeterminable mineral in de- 

 composition. In hand specimens the amorphous rocks appear to be igne- 

 ous, a few scattered crystals of feldspar giving them the appearance of 

 porphyry ; but seen in mass, the water-worn surfaces show with great dis- 

 tinctness, lines of lamination and wave and ripple-marks, which prove 

 conclusively the sedimentary origin of the rock. The beds are inclined 

 15°-20° S. E., the strike being N. 30°-40° E. 



The porphyry of the dykes is evidently eruptive. It consists of a com- 

 pact, amorphous, feldspathic base of a dark chocolate color, in which are 

 scattered crystals of red feldspar, rounded grains of quartz and little 



