TRIASSIC COAL AND COKE OF SONORA, MEXICO 11 



canyons — long, narrow, tortuous, steep-sided, and with many falls. Both are fed 

 by numerous springs and carry water in holes the year round. The Arellanas is 

 much the larger of the two. 



Unlike these creeks, the Calera occupies a large drainage basin, and, with its 

 numerous tributaries, it carries off the water of several square miles. Most of its 

 branches head on the flanks of Candelero or in the ridges which are the continua- 

 tion of that mountain to the north and south. Outside of a few springs of very 

 moderate flow, Calera and its branches are dry, except during the rainy season. 



Calera basin is the coal field proper, and was named from one of the principal afflu- 

 ents of the Calera, the Santa Clara, where the coal was first discovered and worked. 



The rocks of the coal region are Triassic sands, clays, and igneous deposits, with 

 a few later intrusives. A short distance south these are seen to rest upon syenites, 

 referred by the Mexican geologists to the Archean, and on the north we have, 

 near Los Bronces, similar syenites, and a series of interbedded quartzites and 

 granular limestones provisionally referred to the Cambrian. 



The only literature to which I have had access treating on the general geology 

 of this region is the report by Sefiors Jose G. Aguilera and Ezequiel Ordonez.* To 

 the facts there stated I added the results of my own observations in a paper en- 

 titled "Notes on the Geology of Senora, Mexico." f Based on these, the Triassic 

 here is separated into two divisions : 



The Barranca, or clastic sediments ; the Lista Blanca, or igneous rocks. 



The Barranca division is composed of four members. The basal is a series of 

 sandstones and sandy slates ; the second a series of interbedded shales, slates, and 

 sands, with occasionally a band of limestone near the top and with beds of graphite 

 and coal. This is succeeded by a massive sandstone or quartzite carrying pyrites, 

 which often segregate in patches and show strong colorings of iron or copper. 

 The upper bed is a conglomerate or breccia of sandstone with a silicious matrix. 

 This is almost always so strongly altered as to be of the nature of a quartzite, and 

 is seemingly unconformable with the other beds. 



The Lista Blanca division is a volcanic complex, consisting of a series of ande- 

 sitic lavas, agglomerates, volcanic conglomerates, and tuffs, with some rhyolites 

 toward the top. These are found resting directly upon the rocks of the Barranca 

 division, and further north they underlie the Cretaceous deposits. 



The Triassic rocks of the Santa Clara coal field belong to the second series of the 

 Barranca division and to the Lista Blanca. The beds have a general strike north- 

 east-southwest, and an average dip of 30 degrees southeast. The field is separated 

 into two parts by a band of the overlaying Lista Blanca, which obscures the con- 

 nection of the coal beds in the two areas. 



The heavier sands are usually somewhat uniform and persistent, 'but at times 

 they show considerable variabilit^v. They comprise conglomerates, grits, and 

 medium grained sands, their massiveness depending largely on the size of the 

 grain. They are usually gray in color, but may change to brown within a few feet. 

 In hardness they range from friable sandstone to quartzite, but the former condi- 

 tion is rare. They are somewhat clayey at times, and when these clayey sandstones 

 are metamorphosed, as they often are, it is hard to tell which is metamorphic rock 

 and which is igneous. Occasionally the imprint of a branch or trunk of a tree is 



* Contained in Boletin del Instituto Geologico de Mexico, hums. 4, 5, y 6. Bosquejo geologico de 

 Mexico, 

 f Trans. American Institute of Mining Engineers. 



