OF THE SERCaPE-ALAOnAS BASIN OF BUAZIL. 393 



possession points in one direction, namely, that the lithologic characters are not only not 

 constant in these beds, but that they often change very radically withiii a thousand or 

 two thousand feet. 



The oolitic beds, so prominent a feature of the Maroini region, do not appear as such 

 on the Rio Sao Francisco, where the cMitire Sergipe-Alagoas series is cut through by the 

 river. The red sandstones of Penedo do not apjjear as such about Maroim, while they 

 are a conspicuous feature of the geology in the region about Estancia. The shales of 

 Maceio are not seen elsewhere in the basin at any considerable distance from Maeeio, and 

 the arenaceous beds of Villa Nova are not known to exist in any other part of the basin. 



The rocks vary in texture and character between the following kinds : 



1. Coarse, compact sandstone of a bluish-gray color, modified by weathering to 

 brown, and light gray ; fossils preserved but difiicult of extraction. Example, Riacho 

 de Aroeira. 



2. Coarse to fine friable sandstone of open or porous structure; fossils preserved 

 mostly as molds or casts. Example, Coqueiro.. 



3. Oolitic limestone, bluish and greenish, weathering to light gray, cream-colored 

 and brown. Examples, Urubii, Maroim, at the Garabarobe and Lastro quarries. The 

 brown and cream colors of the oolitic beds are apparently products of incipient decom- 

 position, the bluish and green tints predominating where these rocks are quarried to 

 considerable depths, the brown and cream colors appearing near the surface and where 

 the rocks are penetrated by joints or otherwise made accessible to meteorologic 

 agencies.* 



4. Cream-colored, lead-colored and white, soft limestone. Examples, the quarries 

 of Sapucary and Andorinhas. 



5. Limestone, more or less compact, without well-preserved fossils, and with flint 

 nodules. Example, Toque below Canna Brava. 



•The following is a brief description of thin sections of specimens of these oolitic limestones examined under the 

 microscope : 



The body of the rock is made up of irregularly rounded patches of calcite, many of which have the characteristic 

 oolitic structure. There are, besides occasional grains of quartz, a few fragments of grecnisli mica, and here and there 

 patches of brown liydr().xidc of iron. All these constituents are cemented togelher wilh tine, almost colorless, granular 

 calcite. The oolites are stained a dull brown or cream color, and stand out prominently in the cement. There is 

 nothing unusual in the oolites themselves. They are made up of concentric rings of calcite grains one outside the 

 other, the innermost one usually enveloping a small fragment of organic matter, such as a bit of shell. 



Many examples occur among these oolites, however, in which the general mass is net made up entirely of these 

 concentric ring<i, the formation of these having ceased at a certain point, while beyond this the calcite has been added 

 principally upon one side of the already formed mass, and without any definite arrangement. In some cases two 

 partly-formed oolites are cemented by this amorphous calcite, the united mass retaining the usual irregularly-rounded 

 form In other cases still later formed concentric bands have enveloped the oolites thus united. 



A. P. S. — VOL. xvY. 2x. 



