THE SANDSTONE MASSES 



253 



limestone is absent the sand forming the masses was deposited in basins 

 in the Ordovician magnesian limestone at a somewhat lower horizon. In 

 many places these sandston^e masses have been thoroughly protected from 

 erosion by the walls of the basin in which they lie and are covered over 

 by the rocks of later age, the Sylamore sandstone, which is of Devonian 



Figure 2. — Pocket of Saint Peter 8andst077e covered with Sylamore Sandstone. 



age, or the Saint Joe marble, which is Carboniferous. These two sand- 

 stones are quite similar, and it sometimes requires close observation to dis- 

 tinguish between them. A case of this kind is shown in figure 2. 



Sandstone Masses in the manganesian Limestone 



The sandstone masses occurring at the horizon of those mentioned 

 above, while striking to the layman and interesting to the geologist, are 

 easy of interpretation ; but there are numerous others, occurring at a lower 

 horizon, whose origin is not so evident. These masses have a general 

 distribution over northern Arkansas and have been observed by the writer 

 in the extreme southern part of Missouri. Those that are the subject 

 of this paper are confined to no definite horizon, but are liable to be found 

 anywhere in the upper hundred feet or more of the Ordovician dolomites. 

 They vary in size from a few feet to fully 60 feet in height. In some cases 

 they stand up alone on the hillsides, having been uncovered by erosion, 

 and in other cases they are exposed in bluffs, surrounded by limestone. 

 The limestone beds usually abut abruptly against the sandstone masses. 



