XXXVl PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



eiFects produced/' he says, " by tlie agents now operating on the ex- 

 ternal surface of the globe, agencies to which some apply, exclusively, 

 the term actual causes, constitute a term of comparison, which is 

 indispensable to enable us to appreciate the eifects which have been 

 produced by analogous agents in times past. These operations, the 

 effects of the laws of natural philosophy and chemistry, ought ever 

 to be present to the mind of the geologist in his practical observa- 

 tions, because a multitude of circumstances which present themselves 

 in the study of the rocks that compose the earth's surface disclose the 

 operation of some of these agents. Indeed, without we take them 

 into account, it will be impossible to form a just conception of the 

 nature of the greater proportion of rocks, or arrive at a thorough 

 understanding of them." 



THE RECENT PERIOD. 



Before proceeding to notice some of the more important accessions 

 to our knowledge during the past year, respecting geological phse- 

 nomena that belong to the most modern period of the earth's history, 

 some of them operations now in progress, I will offer a few remarks 

 on the terms employed to designate certain geological periods. 



The sense in which the term " recent" is to be understood, as 

 applied to geology, has not yet been defined. All who use it include, 

 of course, the changes now in progress ; but the degree of its ex- 

 tension into time past is by no means a settled point. By some 

 geologists it is confined to the period of which we have authentic 

 records, and has been called the historical period ; by others it has 

 been named the human period, meaning thereby, that it embraces 

 all the time that has elapsed since the creation of the first pair of 

 the human race ; an epoch however which we have as yet discovered 

 BO means of fixing with anything approaching to certainty; and 

 some, as for instance Mr. Darwin in his late work on the Geology of 

 South America, apply the term '' recent" to alluvial deposits con- 

 taining remains of moUusca that are all existing species, but also 

 containing remains of extinct mammalia. Whether any of these 

 animals co-existed with man, that is, with man, not merely as existing 

 in the same country, but then existing on any part of the earth's sur- 

 face, we have no certain knowledge. 



But it is not to the term *' recent" only that this want of precisian 

 is chargeable ; the same uncertainty prevails with regard to the 

 terms by which other geological periods are designated, when used, 

 as they are generally understood, to define a certain division of time 

 in the history of the whole earth. The indefiniteness is perhaps 

 most apparent in regard to the tertiary deposits, and especially in 

 the more modern of these. In the early days of systematic geology 

 there were only two grand divisions of the stratified rocks, the pri- 

 mary and secondary ; the progress of the science called for the sepa- 

 ration of the upper portion of the latter into another grand division, 

 the tertiary ; and its further progress has shown, that there are sub- 

 divisions in the latter that point to great successive changes on the 

 earth's surface, within comparatively modern periods. In 1833 Mi% 



