DISTRICT OF KICHMOND. 393 



shaped holes named " plaster pits," sections of which arc exposed in 

 the cliff; they penetrate both the anhydrite and common gypsum, 

 though they ai'e contracted where they pass through harder portions 

 of the rock, and especially the veins of compact gypsum, some of 

 which are only slightly inclined, and look at first sight like layers of 

 deposition. The pits of which I saw sections have evidently resulted 

 from the percolation of water through the more open parts of vertical 

 joints, and they were cut off where they were intersected by another 

 slightly inclined set of open fissures, which afforded a passage to the 

 water. The accompanying sketch (Fig. 155) shows one of these 

 pits and its relations to the joints and stratification of the gypsum. 



Fig. Lo5.— Plaster Fits. 



c c c 



(«) Gypsum vein. (J) Open joint. (c) Bedding of the gypsum. 



" (6.) Above the gypsum are a few layers of limestone, portions of 

 which appear near the base of the cliff: one of them is studded with 

 tarnished crystals of iron pyrites ; another is a singular mixture of 

 gray limestone and reddish granular gypsum. The portions of lime- 

 stone contained in this rock do not appear to be fragments or pebbles, 

 and they are penetrated by plates of selenitic gypsum. They may 

 be parts of a bed of limestone broken up and mixed with gypsum 

 when in a soft state, or the limestone and gypsum may have been 

 deposited simultaneously and separated by molecular attraction. A 

 rock of this kind is not rare as an accompaniment of gypsum, and it 

 may be merely a result of the mixture of the soft surface of the gypsum 

 with the mechanical detritus first deposited on It. 



" (7.) On the opposite side of the creek, which makes a small break 

 in the section, is a thick bed of marl, Avliose dip a})pcars to be the 

 same with that of the gypsum. In general character it resembles the 



2c 



