134 POSITION OF ARGILLACEOUS DEPOSITS. 



which has been transported from ancient deposits, and is 

 formed chiefly of fragments of older rocks. The most 

 important accumulations of drift in this country are divided 

 into three groups: the first, the Silurian, which overlies 

 the district of that name, and is of local origin, consisting 

 of fragments of the rocks of that formation ; the second 

 extends southward from Lancashire, over Cheshire, Stafford- 

 shire, "Worcestershire, Gloucestershire, and other midland 

 counties, and is composed of fragments of crystalline and 

 trappean rocks, which constitute the mountains of Westmore- 

 land, Cumberland, and part of Scotland; while the third 

 comprises the various deposits which strew our north-eastern 

 coasts and the adjacent districts with fragments of rocks 

 analogous to those now existing in Scandinavia, and fur- 

 nished either from that region, or from land which once 

 occupied the present bed of the German ocean. 



ARENACEOUS DEPOSITS. Sandstone is an aggregation of 

 minute particles of quartz ; it is usually cemented by a sub- 

 stance of like nature with itself, or the cement may be of the 

 nature of lime, clay, or iron. When sandstone is of extremely 

 coarse grain, it is termed grit. 



ARGILLACEOUS DEPOSITS. The substances comprehended 

 in the term clay scarcely admit of a general description ; but 

 most of them agree in possessing an earthy texture, and 

 emitting an argillaceous odour when breathed on. They 

 consist of silica, with a variable proportion of alumina, 

 and a small quantity of lime or magnesia. The term is 

 also extended to all kinds of indurated mud, derived from 

 the decomposition and abrasion of various rocks. 



POSITION or ARGILLACEOUS DEPOSITS. It has frequently 

 been mentioned that the lower beds of many formations are 

 argillaceous. This is peculiarly the case in the oolitic series ; 

 and is probably owing to the fact, that the alluvial cliffs, or 

 shores, then in existence, being the first which yielded to the 

 action of the waves, provided materials for the earliest strata 

 of the new era ; while rocks of firmer texture yielded more 

 slowly to the abrading force, and furnished, at a later period, 

 the material for those arenaceous and calcareous deposits 

 which occupy higher positions. 



CALCAREOUS ROCKS. This term comprehends all the series 

 of rocks of which lime forms a prevailing element, including 



