walcott.I SUMMARY TEXAS. 355 



A second section, taken about 1 mile south of the preceding, is also 

 quoted, as it carries the series down to the granite : x 



Feet. 

 No. 1. — Cretaceous strata in horizontal beds 110 



No. 2. — Slope, covered with conglomerate, composed of pebbles of sandstone and 



calciferous sandrock, rather firmly cemented* 12 



No. 3. — Potsdam sandstone, consisting of thin layers of variegated green, gray, 

 and purplish sandy limestone, with bands of dolomite and silico-cai- 

 careous rock interstratified. Some of the beds are highly charged with 

 trilobites, of which the most common are Arionellus texanus, Dikeloce- 

 phalus roemeri, and Conocephalites hillingsi 40 



No. 4.— Highly ferruginous siliceous sandstone, composed of fine grains, loosely 

 cemented, passing downwards into coarse gritstone and conglomerate. 

 The upper beds contain Lingula and Obolus (?) 60 



"No. 5. — Flesh colored granite, interstratified with veins of milky quartz 6 



From the sections it will be observed that the strata consist largely 

 of limestone, with a bed of sandstone at the base. 



The section of Packsaddle Mountain, in Llano County, in the valley 

 of Honey Greek, is given as follows by Mr. C. D. Walcott in 1884 : 2 



Feet. 



5 = Potsdam limestone 60 



4 = Potsdam sandstone 30 



3 = Potsdam limestone 310 



2 = Massive Potsdam sandstone 205 



1 = Llano group. 



605 



The fauna collected included 12 genera and 25 species, all character- 

 istic of the Upper Cambrian zone. 



A comparison of the Texas section with that of the Grand Canon of 

 the Colorado, in northern Arizona, leads to the conclusion that they 

 belong to the same area of sedimentation. In each a massive sand- 

 stone at the base is subjacent to alternating sandstones and limestones 

 that carry essentially the same fauna. The sections of the lower sand- 

 stones of the eastern slopes of the Rocky Mountains partake somewhat 

 of this character, especially those of the Big Horn range in Wyoming. 

 In Wisconsin, etc., the calcareous beds of the Texas section are absent, 

 but the presence of the Dikelocephalus fauna at the summit of each 

 series above a zoue carrying what may be called the Ptychoparia minor 

 fauna, proves that the life of the two areas continued on in the same 

 succession despite the change in sedimentation and environment. 



In a report ou the Cambrian system Prof. T. B. Comstock concludes 

 that at least two and probably three series of Cambrian strata are 

 represented in central Texas. 3 To the basal beds he gives the name 

 Hickory series (Lower Cambrian 1). For the second horizon the name 

 Riley series (Middle Cambrian t), and for the upper series the name 



' Op. cit., p. 217. 



2 Note on Paleozoic rocks of central Texas. Am. Jour. Sci., 3d ser., vol. 28, 1884, p. 432. 



3 A preliminary report on the geology of the central mineral region ot 1st Ann. Rep. Geol. 

 Surv. of Texas for 1889-90, pp. 285-291.. 



