GEOLOGY OF THE MESOZOIC AEEAS. 7 



areas) show the granitic grits that are so conspicuous in them. The beds 

 are usually barren sandstones and shales, formed of well-sorted components. 



The lowest group seems to correspond to a period of rather slow subsi- 

 dence and slow accumulation of sediment. In the period of the formation 

 of the middle member the conditions seem to have favored the growth of 

 vegetation, perhaps because the subsidence was slower. The coal-bearing 

 areas seem then to have been in the condition of a marsh. During the depo- 

 sition of the upper group the sinking of the areas seems to have been more 

 rapid, and the action of the water to have been sometimes quite violent if 

 not aided by ice in some localities. 



Owing to the extensive explorations for coal, the geology of the Rich- 

 mond Area is much better known than that of any other. It is of much 

 more importance, as this area gives us nearly all the older Mesozoic plants. 

 It may also be taken as typical of the geology of the other areas. A few 

 details will be given now of the geological structure of this field. 



As stated before it has a synclinal structure, but many facts go to show 

 that it did not possess this structure in its early history in such a marked 

 manner as now. It, like the other areas, was a progressively subsiding 

 region, probably, during most of the era of deposition. 



The strata forming the lower group in the Richmond Coal Field are 

 mostly sandstones, rather coarse in texture, and sandy shales. They are 

 often much indurated and affected by "slickensides" and small local dis- 

 turbances. The lowest sandstones are not easily distinguished from the 

 underlying granitoid gneiss. The thickness of these beds varies much 

 with the locality in which they occur. It ranges from less than 100 feet 

 to 500 or 600 feet. These rocks are mainly of a gray color. The middle 

 group varies in thickness from 100 to 200 feet. Here a large proportion of 

 black shale occurs, some of which is very fine grained and so much indu- 

 rated as to approach in nature a slate. Both these beds and the included 

 coal show compression, local disturbances, "slickensides," &c, but in a 

 less degree than the lower group. The number of the coal beds, their 

 thickness, and their quality, vary in different parts of the field. Usually two 

 independent and persistent seams are to be found. Sometimes for a short 

 distance above and below these a number of smaller beds occur, but these 



