318 C. C. MOOK ORIGIN AND DISTRIBUTION OF THE MORRISON 



a certain specimen should be called calcareous shale or argillaceous lime- 

 stone. Many of the "joint-clays," when examined Avith the microscope, 

 are seen to be exceedingly fine-grained sandstones, sometimes with a 

 matrix of hematite, and not clay at all. True kaolinic clays do, however, 

 occur in some abundance. The colors of the formation vary to a great 

 degree, giving rise to the term variegated beds, often applied to the for- 

 mation. The clays are, in places, brick red or chocolate colored, due to 

 the presence of large amounts of hematite ; at other places they are gray, 

 white, purple, or nearly black. The sandstones are usually yellow or 

 white, but may be reddish. They are often made up largely of angular 

 or rounded quartz, as the case may be, witli replaced feldspars, calcite, 

 and minor amounts of volcanic matters and otlicr material. Tbe lime- 



A 



D 



Figure 3. — Diug rum matte Representation of the Thickness of the Morrison Formation 

 in various Areas from Southwest to Northeast 



A, maximum thickness in tlie Telliiride quadrangle, Colorado ; B, thickness in the 

 Encampment district, southern Wyoming; C, thickness at Como Bluff, Wyoming; D, 

 thickness near Devils Tower, Wyoming. Scale, 400 feet to 1 inch. 



stones are usually thin, are often fine grained, and generally argiUaceous. 



Perhaps the most characteristic features of the Morrison are the varie- 

 gated colors of most of the outcrops, the "uniformly variable" character 

 of the succession of beds, and the presence of distinct channeling, witli 

 sandstone lenses occupying depressions in the uiideilving clays. 



The origin of the j\r()i-ris()n formation has been the subject of a number 

 of discussions in the past. Some workers have held that the beds weiv 

 deposited in tbe sea; others, such as C. A. White, have held that tbe 

 formation was deposited in a great lake. Kiggs has advanced the theory 



