THE OLDER MESOZOIC OF ARIZONA. 21 



then into true sandstones. These sandstones are of a light color, con- 

 trasting strongly with the dark-brown sandstones of the Moencopie 

 already described. They are, moreover, always more or less cross- 

 bedded and usually exhibit hnes of pebbles running through them in 

 various directions. These are true sandstones, very hard, devoid of 

 alumina, and scarcely affected by the winds, so that their angles are 

 usually sharp and the ledges they form are abrupt and jagged. Although 

 the sandstones proper generally occur lower down, there is no uniformity 

 in this arrangement, and sandstones are often found in the middle and 

 conglomerates more rarely at the top. But in addition to these the 

 Lithodendron member embraces other classes of beds. There is a well- 

 stratified layer of thinnish sandstone shales that is often seen imme- 

 diately under the heavy sandstone cap. Some of these shales have a 

 grayish color and are highly argillaceous. These layers tend to thicken 

 even within the member itself, but especially farther out, and, what 

 is more significant^ they often become transformed into a bluish white 

 iparl. This condition can be seen between the beds of conglomerate 

 in places where the Lithodendron beds are comparatively thin, as in 

 the lower valley of the Little Colorado, where they are only about 300 

 feet in thickness. This feature is not very prominent, but at other 

 places, as in the Petrified Forest region, where the Lithodendron beds 

 attain their maximum thickness of 700 or 800 feet, this tendency on 

 the part of certain beds to become transformed into marls is the most 

 marked feature of the member. The marls here occupy much more 

 than half of the beds. They are very varied in color, showing besides 

 the white and blue tints a great variety of darker ones, ' such as pink, 

 purple, and buff. These heavy marl beds, of which there may be sev- 

 eral in the same cliff, are interstratified between conglomerates, coarse 

 gravels, and cross-bedded sandstones, all of which taken together form 

 the beautifully banded cliffs that are seen throughout the Petrified 

 Forest, especially along its northern flank. It thus becomes necessary 

 to include under one designation all of these varying beds, which often 

 change the one into the other even at the same horizon within short 

 distances. 



It remains to mention certain minor features, which are not uni- 

 versal, but which nevertheless have considerable importance. In the 



