234 MANUAL OF AGPJCULTUEE. 



water, e.g., rock salt ; and organically by tlie agency of organised 

 living structures, e.g., coal and peat, both of wliicb are the 

 remains of plant life, and limestone, the remains of minute 

 animalcuh^. Stratified rocks have a threefold classification, 

 denoting the epochs of their respective formations, viz, the 

 Primary or Paleozoic — ancient ; Secondary or Mesozoic — middle; 

 and Tertiary or Cainozoic — modern. The expression primary; 

 signifies no fixed era or standpoint of time, — merely that indefi- 

 nite portion of the past when the first sedimentary rocks began 

 to be deposited beneath the water. By " era," too, the geologist 

 understands any period comprehending groups of living organisms 

 bearing jDoints of close resemblance to each other. The names 

 oiven to the different formations have c^enerallv been taken from 



CI? O «y 



that of the most characteristic or useful rock in the group, which 

 may sometimes include a stratum, of quite an opposite texture 

 and composition. Thus the old red sandstone formation includes 

 some of the densest clay rocks, and it may appear contradictory 

 to apply the term sandstone, clearly suitable in one district to 

 the clay slate of another. But as these two contain the identical 

 class of fossils, their similarity of age is demonstrated, and hence 

 the justice of their sharing the same family appellation. 



The stratified rocks are arranged in the followiug leading 

 groups, in order of tim e : — 



I. Those of the Primary or Palceozoic period, including 



(«.) The Laurentian or Pre-Cambrian era rocks, which are 

 principally composed of gneiss, a metamorphosed granite, 

 Vv'hose original granite particles have been disintegrated 

 and redeposited, and compacted with a different structure 

 and texture. 



(l.) Cambrian era rocks, composed of grits, slates, and con- 

 glomerates. 



(c.) Silurian era, divided into (1) lower, comprising the Lin- 

 gula Llandeilo, and Caradoc beds, and (2) the upper divi- 

 sion, comprising the Llandovery, "VYenlock, and Ludlow 

 beds. 



{d.) Devonian and Old Pied Sandstone era, comprising the 

 lower, middle, and upper Devonian beds of England, and 

 the like three Old Bed Sandstone beds of Scotland. 



(c.) Carboniferous. — Carboniferous or mountain limestone, 

 millstone grit, and the coal measures. 



(/.) Permian — the lower, containing red marl, sandstone, and 

 conglomerate, and upper containing lower and upper mag- 

 nesian limestone. 



II. Secondary or Mesozoic yeriod, conta.ining 



{a.) Triassic, or Kew Bed Sandstone, of lower, middle, and 



upper formations. 

 (l.) Jurassic era, embracing Lias (lower, middle, and upper) ; 



