1C4 



The three primaiy geological life-periods are arranged 

 into eight systems, each distinctly marked lithologically 

 and palfeontologically. These systems represent many 

 long epochs of om* earth's history ; and thongh in 

 some instances long intervals may have elapsed between 

 them, yet each may be expected to afford us a very 

 correct knowledge of a vast continuous length of time. 

 The systems are again divided; and those divisions, 

 most important and uniform in character, have been intro- 

 duced into the illustrative table. To have detailed all 

 such subdivisions woiild have extended the subject be- 

 yond due limits. The classification of the vegetable 

 kingdom into five classes, and that of the animal kingdom 

 into nineteen classes, includes all existing plants and 

 animals. All these classes have been recognized in the 

 Cainozoic period ; and all fossil organisms that have yet 

 come to light, can, with propriety, be placed under some 

 one of the twenty-four classes adopted. The table is 

 arranged so as to indicate the classes present in each 

 system ; and the diagram conveys an idea of the relative 

 thickness of each of the representative recks, and perhaps 

 a very accurate one of the time occupied in formation, 

 supposing, of course, the rate of deposition to have been 

 uniform. 



The earliest Flora we are acquainted with is that of 

 the Lower Silurian ; it consists of marine aquatic plants,* 

 Thallogens ; that humble class being absent in the Cam- 

 brian system. The first trace of a land plant is a Lepidos- 

 trobus — an Acrogen, at the very top of the Upper Siluiian. 

 "We may, therefore, conclude that there were no land 

 plants during the long Silmian epoch, a vast interval, far 

 exceeding in duration any other system. In the Devonian 

 system, in addition to Thallogens and Acrogens, are foimd 

 the first trace of Gymnogens ; but the remains of 



* Algse, 



