GEOGNOSY AND GEOLOGY. 177 
and yellow, whose uniformity is interrupted by dark spots, cloudings, veins, 
&c. A slight tendency to green is produced by the presence of a little 
chlorite. The chalk marl exhibits various shades of color, but not the 
markings so peculiar in the preceding ; it is generally white, green, grey, or 
yellowish. The variegated clay marl, and marl clays, often exhibit a great 
similarity to those of the rock salt formation, and contain nodules and entire 
veds of fire-stone. 
The limestones, often marly. are white and grey, more rarely blue and 
greyish black ; the purer varieties have a conchoidal brittle fracture, and 
are frequently traversed by veins of calcareous spar; they are sometimes 
exhibited as marble or oolitic limestone, and are accompanied by clay or 
clay marl. 
Subordinate masses are an iron-stone which is frequently mixed with 
green earth, gypsum, karstenite, and rock salt, all, however, only occurring 
in the marl. 
b. White Chalk Group. This, also, is divisible into two formations, the 
lower of which is of most frequent occurrence. It is in this that the true 
chalk exists. The white color often passes into grey, reddish, or yellow, 
and the hardness varies from that of the chalk of commerce to that of 
compact limestone. _ Fire-stones frequently occur in nodules lying parallel 
to the planes of stratification, or in entire beds produced by the aggregation 
of the former. It consists,in great measure, like the chalk itself, of 
infusorial skeletons, recognisable by a good microscope. 
Chalk rock comes near to the chalk proper, and consists of a white 
silicious limestone. The silex is not unfrequently separated in the form of 
nodules. Here belongs also the flag limestone, a compact rock of brittle 
fracture, and sometimes mixed with sand. The accompanying chalk mazls 
are chalk white, and pass on the one hand into chalk, on the other into 
chalk rock. Dolomite is very rare; clay and loam beds occur between 
strata of firmer texture. 
The upper formation is more seldom met with, and consists of Saugkalk. 
chalk (in Petersberg, near Maestricht) and a granular saugkalk of ochre 
yellow color. In these rocks there also occur nodules of freestone and of 
hornstone. A great variety of fossils is met with, among others, Lacerta 
gigantea, and Pagurus faujasii. 
Above these limestones lie fossiliferous, earthy, sandy marls, of white. 
grey, yellow, green, or brownish colors. Clay and loam beds colored by 
greensand or oxyde of iron, occur as subordinate masses; also, a marly 
sandstone mixed with grains of earthy chlorite, a calcareous sandstone with 
many granules of lime, a calcareo-silicious conglomerate with fragments of 
shells, and a marly limestone. 
The cretaceous formation is developed most completely in England, and 
has there been investigated most fully. A section of the stratification is 
exhibited in pl. 46, fig. 17. Beneath lie the strata of the Jurassic fluviatile 
formation; reckoned by many geologists among the cretaceous: they 
include the strata, 1, 2,3. Upon these rests the lower formation of the 
first group : first the lower greensand, 4, with the gault, 5 (the blue clay bed) : 
607 
