§1. INTRODUCTORY REMARKS. 193 



our globe, and on investigations of the natural records of 

 similar changes in periods antecedent to all human history 

 and tradition, the minerals and fossils are the alphabet, the 

 rocks and mountains the volume, by which the student of 

 this interesting department of science must learn its im- 

 portant lessons. But to those who cannot examine Nature 

 in her secret recesses, or accompany an experienced teacher 

 to the valleys, or the mountain-tops, lectures illustrated by 

 specimens and drawings, afford, perhaps, the best substi- 

 tute for the more efficient and interesting mode of in- 

 struction. 



That we may obtain a clear and comprehensive view of 

 the vast field of inquiry that lies before us, artificial classi- 

 fications are necessary in this, as in other departments of 

 science ; and without assuming that the arrangement in 

 which the various deposits are grouped by modern geologists, 

 may not require considerable modification as new facts are 

 brought to light, I will place before you a tabular view 

 of the formations, or systems of deposits, in their pre- 

 sumed chronological order. At the same time it must be 

 borne in mind, that all arrangements of this kind necessarily 

 involve arbitrary distinctions ; and that it may, possibly, 

 hereafter appear, that we have in some instances classed as 

 general, what are but local phenomena ; and have grouped 

 together in one system, strata, which farther investigations 

 may show to be distinct formations, separated from each 

 other by vast periods of time. It must too be admitted, 

 that there are not in nature those strict lines of demarcation 

 between one system of strata and another, which, for the 

 convenience of study, have been adopted. For though 

 the limits of certain formations may be clearly defined 

 in one region, in other and distant countries there may 

 be an insensible passage from one system to the other. 

 In short, both in the organic and inorganic kingdoms 

 of nature, in proportion as our knowledge is increased, 



o 



