SILURIAN. 227 



(J-H 12. SONOBA, MEXICO. 



Aguilera ® in 1906 mentioned certain strata as " doubtful Silurian" in describing 

 under that head "Quartzites and fossiliferous semicrystalline limestones, of gray 

 color and siliceous character, with Encrinites cyaihophyllum and Heliolites; these 

 overlie compact gray and yellowish limestones which occur above a series of breccias 

 and limestones." Aguilera does not name localities at which these rocks occur. 



The areas mapped as Silurian in Sonora are so assigned on authority of a manu- 

 script map furnished by Robert T. Hill, and the strata may be similar to those 

 described by Udden and Richardson as occiuring in southwestern Texas. 



H 13. WESTERN TEXAS. 



Describing a region near Marathon, Tex., J. A. Udden ^'^* says: 



There must be several thousand feet of sediments exposed in this area. They have been 

 folded and faulted and dip, usually at high angles, either to the southeast or to the northwest. 

 * * * In the south part of the area they consist largely of dark shales and limestones. These 

 contain heavy ledges of cherty quartz, which rise in long ridges running from the northeast to 

 the southwest. The chert is usually interbedded with ledges of dark limestone, which now and 

 then contain round and loaf -like concretions of chert. The prevailing color of the chert is white, 

 but it is sometimes black, green, bluish, or of a brown or a pinkish color. With some of the 

 limestones there are thin seams of sandstones and conglomerates. The latter usually consist 

 of well-worn pebbles, mostly of limestone, but also of hard quartzite. In some shales, which are 

 associated with chert and limestones in a ridge on the south side of Edwards Creek, about 10 

 mUes northeast of Santiago Peak, there is one stratum which contains large concretionary aggre- 

 gates of crystals of barite. Some of these aggregates are a foot in diameter and consist of 

 columnar crystals radiating from the center of the concretions. The mineral was seen to 

 follow a stratum about 3 feet in thickness, and the concretions in one place appeared to be 

 present in large quantity. The limestones themselves are frequently bituminous. Some 

 ledges consist of small worn organic fragments. From one of these some fossil fragments were 

 taken which Dr. Charles Schuchert has identified as the outer portions of the glabella of a 

 Trinucleus. Associated with this there were also a Plectambonites {sericeusV), a Rafinesquina, 

 and possibly a Zygospira. A gastropod of the genus Cyclora was found in another ledge, and 

 in stUl another limestone a Nodosaria was noted. All of these fossils were collected along the 

 wagon road near Ridge spring and at different points south from this place for a distance of 10 

 miles. The same kind of rocks continue north from this spring as far as to Pelia Colorado, 

 4 mUes south of Marathon. North from Pefia Colorado we find shales, limestones, and thin 

 conglomerates, but no chert beds. An Athyris, a Fistulipora(?), a Dentalium, and joints 

 of crinoid stems were observed in the vicinity of Marathon, where the dip of these rocks is quite 

 generally to the northwest. Evidently these sediments are younger than the formation which 

 contains the chert south of Pena Colorado. . 



The thickness of the ancient sediments, which are seen on these plains around Marathon, is 

 no doubt several thousand feet, and they very likely contain formations of more than one age. 

 For 25 miles the road from the Chisos runs over the edges of beds tilted at high angles. The 

 fossils associated with the chert show that some of these strata belong to the Ordovician, and 

 Dr. Schuchert infers that these are of the Trenton period. In his "Physical geography of the 

 Texas region" Prof. R. T. HiU expresses it as his opinion that the limestones, shales, etc., of these 

 plains may be of Lower Helderberg age," and there is no good reason to doubt that the Silurian 

 rocks are represented in this extensive complex of folded strata. 



2 Folio 3, Top. Atlas U. S., U. S. Geol. Siirvey, 1900, p. 4. 



