GEOGNOSY AND GEOLOGY. 187 
sand and mica. Shells occur in great number. The principal foreign 
minerals are sulphur, iron pyrites, arsenical pyrites, gypsum, and petroleum. 
Limestones, which in general are very subordinate ; they are met with as 
compact, slaty, loose, oolitic, marly, and breccious; as also marly lime, 
which is often bituminous. Likewise fetid limestone, silicious limestone, 
sandy limestone, and a calcareous conglomerate. 
Among the true subordinate masses belong gypsum, often in the form of 
purest alabaster, rock salt, lignite, not unfrequently inclosing amber, mellite. 
humboldtine, and retinasphaltum. Iron and arsenical pyrites often occur, as 
also silicified wood; likewise a granular, argillaceous, or sandy iron-stone. 
Carbonate of manganese or rhodocrosite is occasionally met with. 
The metalliferous sands of the molasse are of great importance, the metals 
being disseminated in the form of fine grains, as of platinum, gold, magnetic 
iron-stone, titanic iron, chrome iron-stone, tin ore, &c. These are obtained 
by stirring and washing the sands in the water, when the metallic matters: 
fall to the bottom by reason of their greater specific gravity. Diamonds are 
sometimes obtained in a similar manner. 
This formation is divisible into two groups : 
a. Lower Group, or the Marl Formation. The principa] mass consists of 
clay, sand, and marl, among which occur sandstones and fresh water lime- 
stones. Fossils exist in this formation, generally similar to those of the present 
day. There are numerous genera of Pachydermata, or thick-skinned mam- 
malia. Beds of brown coal, of great extent and thickness, are accompanied 
by clay or sand, or, instead of the latter, by sandstone, converted into quartz 
grit in the neighborhood of abnormal masses. In these brown coal beds, and 
especially in the lower portions, upright stems of trees are met with. The 
brown coal experiences considerable modification in the vicinity of those 
abnormal masses, generally basalts, which penetrate them. The nearer to 
the latter, the greater the similarity to stone coal; and at the surface of 
contact the coal is changed into a lustrous coal, disposed in short columns 
perpendicular to the abnormal mass. These beds sometimes become 
inflamed by the oxydation of some of the included mineral substances, and 
thus exert an igneous action on the rocks with which they are in contact. 
In this manner the porcelain jasper is produced from clay masses. In the 
vicinity of the brown coal there sometimes occurs polishing slate, or tripoli, 
consisting entirely of the shields of infusoria. Leaves and fruits (Phyllites 
and Baccites), especially of Thuja, Juglans, Salix, Populus, Betula, and Acer, 
are found in the coal, but are specifically distinct from any of the present 
epoch. Remains of fishes and reptiles are also embraced in this group. 
The region of Mayence furnishes an illustration of local variation in the 
marl group, a section of which is presented in pl. 46, fig. 18. Beneath lies 
a blue marl clay (1); next to this come sea sand and conglomerate, with 
numerous remains of Cetacea and Plagiostomes; then a brackish-water 
limestone, or one containing both marine and fluviatile shells, with Mytilus 
faujasii and Cerithium plicatulum (2-7); above the whole are sand and 
sandstone masses, in which lie imbedded numerous remains of terrestrial 
mammalia. 
617 
