114 Scientific Geology. 



geological epoch, or period.* The second group embraces only 

 gneiss, and those rocks which are so intimately associated with it that 

 they constitute but a single formation. The third group comprehends 

 mica slate and those rocks that are so closely connected with it, as to 

 show great similarity in the causes that produced them ; although per- 

 haps not all of them, were formed during the same epoch. The fourth 

 group is miscellaneous, including rocks that have no necessary con- 

 nection or resemblance. The fifth group includes all the consolida- 

 ted rocks resulting from sediment ; although obviously belonging to 

 at least three distinct formations. The sixth, or tertiary group, takes 

 in all the unconsolidated beds above the chalk, or its equivalent in 

 this country, the ferruginous sand formation. 



The tablets attached to the map will show the particular marks by 

 which the members of the different groups are distinguished from one 

 another : And to afford still farther means for accomplishing the same 

 object, and preventing mistakes, I have placed a figure on each tablet, 

 which corresponds with the same figure placed upon the map in eve- 

 ry place occupied by that particular rock ; so that even if in any case 

 the painter has applied a wrong color, these figures will afford the 

 means of detecting- the mistake. 



STRATIFIED ROCKS. 



The uppermost portion of this division of the crust of the globe 

 consists for the most part of unconsolidated layers of sand, clay and 

 gravel. The lower portion embraces all those solid rocks that are 

 divided by parallel and continuous seams. The stratified rocks oc- 

 cupy in every country by far the largest proportion of the surface. 



ALLUVIUM. 



It is well known that a number of causes are daily operating to 

 modify the surface of the globe. In some instances new and solid 

 rocks are gradually forming ; in others, and those far the most nu- 

 merous cases, the rock strata are wearing away, and the fragments, 

 carried by water to the lowest spots, are deposited in the form of sand, 

 grave], clay, and loam. But all such deposits, whether consolidated 

 or not, are denominated alluvium ; excepting only the products of 

 volcanoes. 



* See Brongniart's "Tableau dea Terrain's" &c. p, 4 Introduction. 



