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The strata, for a, few feet on each side of a dyke, are usually consid- 

 erably altered, being hard and flinty, while at the same time they 

 are tilted upward at a more or less strong angle, as if the rent had 

 been widened, not by a horizontal movement of the beds, but by 

 the bending upwards of the strata on both sides of the fissure, 

 through the force of the extruding matter. Sometimes in the denu- 

 dation of the surface, these dykes, as just remarked, project like 

 ruined Avails, while at others, with the hardened strata on each side, 

 they form low ridges, that run, sometimes for long distances, on the 

 surface of the campo. 



In the village and immediate vicinity, there are no good rock 

 exposures. The most interestiug locality, and by far the best col- 

 lecting ground for fossils, lies at a distance of about two miles 

 to the northward, in a large, open, treeless, grassy campo. The 

 surface here is quite undulating, and strewn with angular frag- 

 ments of a red or whitish sandstone, rarely ever seen in place. 

 In the rain-courses the rock exposed is usually a fine, soft, well 

 laminated, whitish or yellowish shale, usually quite unproductive 

 in fossils. From the yellow shale I have obtained only a large 

 Lingula, fragments of VittiUna pustulosa Hall, nob., showing the 

 imprints of the little spines and a single ventral valve of a Bpir- 

 ifer. This shale, which I know only in a somewhat decomposed 

 state, is largely made up of minute silicious particles and little 

 mica flakes. It takes excellent casts of fossils, and would proba- 

 bly repay more careful examination, but I was unsuccessful in my 

 search for a good exposure. 



The great repository of fossils is the sandstone, which, as on the 

 eastern side of the igarape de Erere, appears to form bands, a few 

 inches in thickness, interstratified with the shales in their upper 

 part. On the washing out of the shales by water the sandstone has 

 cracked up and been left lying in fragments on the surface. Fossils 

 were collected from the loose fragments, but, on the summit of a 

 low ridge, to the north of a deserted house, I discovered on my last 

 visit a layer of the sandstone, which, with great labor, Mr. Derby and 

 I succeeded in uncovering ; and this yielded us a magnificent lot of 

 fossils. The layer is only about four inches in thickness, but it is 

 completely filled with fossils which are usually in the shape of moulds, 

 the organic matter having been entirely removed. The rock is com- 



