SOUTHERN PACIFIC LINES 87 
passes onto strata of Paleozoic age, which underlie the limestones and 
shales of the Trinity group. The Paleozoic rocks exposed near 
eae siding are sandstones and shales of the Tesnus fottiatSon) of 
nnsylvanian age, of which this is the type locality. It gives rise 
a rounded hills, on which are many russet-brown outcrops of the 
sandstones. 
The Marathon Basin, which is entered near Tesnus, is an area of 
plains and low ridges about 40 miles wide, in which rocks of Paleozoic 
age are extensively exposed.” It is surrounded on all 
Tena sides by escarpments of the limestones of Cretaceous 
ve age which formerly extended entirely across the region 
ut have been removed by erosion from the area of 
the Marathon Basin. The basin occupies the truncated crest of the 
® The following table and description of the Paleozoic formations exposed in 
the Marathon Basin are furnished by P. B. King: 
Age Formation bert Character 
4 cae Limestone, panterbedded with Le fees gei ga 
aptank formation. . Marathon 9 on n Side 0 
Marathon Basin, w! a overlain by the 
Thin-bedded sandstone and shale, with thick beds 
Haymond formation. 2, 500 a boulder —— upper part 
Pennsylvanian Haymond siding. 
series. 
Dimple limestone. 400-1, 000 oer pacar gear shale, cropping out in 
Thick-bedded brown sandstone, with some inter- 
Tesnus formation. 7, 500 bedded shale, with a rg of ge shale at the 
base several thousand feet 
200-600 N dg and chert, which crop out in prominent 
Devonian (?) 
system. Caballos novaculite. 
Maravillas chert. 100-400 | Black chert and limestone. 
Drab shale, with ee flaggy brown sand- 
Woods Hollow shale. 500 stone and limestone 
eres Massive limestone, bedded reddish chert, and some 
Ordovician sys- | Fort Pefia formation. 150 conglomerate. 
Alsate shale. 50 | Shale and thin limestone beds. 
gray limestone, with a thin medial member 
Marathon limestone. 500-800 woh chery Raccuciee, much shale and some con- 
Cambrian sys- ; Shale with thin brown limestone and sandst 
tem. Dagger Flat sandstone. 300+| “beds, passing down into massive Pare at 
The Cambrian and Ordovician rocks | fossils are also different. The Ordo- 
differ considerably from those in the | vician rocks at Marathon contain 
region of Van Horn and El Paso, only a | chiefly graptolites and linguloid brachi- 
few hundred miles away, where the | opods, animals which lived in an en- 
rian cephalo , gastro 
is much sandstone, shale, and conglom- | sponges, which lived in a clear sea 
erate in addition to limestone. The | where limy sediments were being 
