THE LEWIS SHALE 607 



variations previously noted in the upper and lower limits of the forma- 

 tion, the coal-bearing rocks of all of the fields described in the Rio 

 Grande Valley are lithologically, stratigraphically, faunally, and florally 

 identical with the Mesaverde of the San Juan Basin. 



LEWIS SHALE 



The Lewis shale is known to occur in the region described in this paper 

 only in the San Juan Basin. It is 1,600 feet thick near Durango, on the 

 northern rim of this basin, and possibly thicker in some other places. 

 According to Mr. Schrader (100, page 242), its maximum thickness is 

 2,500 feet. It thins rapidly southward, and at its outcrop, in the south- 

 ern portions of the basin, is about 200 feet thick (116, page 171). Doctor 

 Gardner states (110) that it is 2,000 feet thick near Gallina, but that in 

 Arroyo Torreons, about 30 miles southwest of Gallina, it is only 250 feet 

 thick. No explanation of this great variation in thickness can be given, 

 but the suggestion previously noted seems pertinent that the Lewis may 

 become sandy and coal-bearing to the south, and thus be equivalent in 

 age to the upper portion of the Mesaverde of the Cabezon region. In this 

 connection it may be noted that a shale of marine origin nearly twice as 

 thick as the Lewis of the Arroyo Torreons section occurs in the midst of 

 the Mesaverde in the Rio Puerco field, and that a sandy shale of marine 

 origin of nearly the same thickness occurs in the Tijeras field above the 

 principal group of coals. Because the fossils from these shales do not 

 differ notably from those of the coal-bearing rocks above and below them, 

 the shales are regarded as probably parts of the Mesaverde formation. 

 However, it is entirely possible that when the faunas and floras of this 

 region are better known some of the rocks here described as Mesaverde 

 may prove to be equivalent in age to a part of the Lewis shale. 



Few fossils have hitherto been collected from the Lewis shale of the 

 southwest. Baculites asper (73, page 5) was found in it in southwest 

 Colorado, and Doctor Gardner found a few fossil shells in it southeast of 

 Raton Springs (locality number 4455 of the accompanying map). These, 

 together with the shells collected by the present writer near Dulce (locali- 

 ties numbers 7200 and 7201), seem to make rather definite the correla- 

 tion of the Lewis shale with the top of the Pierre of the Raton coal field, 

 east of the Rocky Mountains. The fossils are named and their relation 

 to those collected in the Raton field indicated in the table of invertebrates 

 previously given. 



"LARAMIE" FORMATION 



The "Laramie" formation occurs within the area described in this 

 paper only in the San Juan Basin. It is more than 1,000 feet thick near 



