ALLUVIUM. 55 



The first group embraces all those loose deposits that occupy the surface, and whose 

 description comes under the name of Surface Geology excepting the Tertiary strata. 

 As these lie open to the inspection of all, and their position is not doubtful, their descrip- 

 tion may form a convenient introduction to the older rocks. In the second group we 

 have a series of rocks commencing with the oldest known on the globe, whose relative 

 age and order of superposition are clear, up to a certain height, and admitted by all 

 geologists, and therefore these seemed to claim a description before the third group, which 

 embraces rocks whose lithological characters are generally well marked, but whose true 

 position in the geological scale is as yet quite uncertain. By this arrangement we begin 

 with formations well known, and advance towards those over which much doubt and 

 obscurity still hang. The fourth group is natural, embracing the decidedly unstratified 

 rocks. 



PART I. 



ALLUVIAL AND TERTIAEY ROCKS. 

 1. ALLUVIUM. 



Under this term we include all the loose or partially consolidated materials, that have 

 been worn from the older rocks at whatever period, and brought into their present state 

 since the tertiary period. These materials, by whatever agencies first torn off from the 

 solid ledges, have been mostly more or less sorted and deposited by water in layers or 

 strata generally horizontal. The size of the fragments varies from that of enormous 

 blocks, weighing thousands of tons, down to the impalpable powder of the finest mud. 

 In some instances certain ingredients have been dissolved out of the general mass, and 

 separated by water, until evaporation has left them in the form of marl, bog iron and 

 manganese ores, &c. These, also, belong to alluvium, and so does a consideration of all 

 the geological agencies that have operated on the earth's surface since the tertiary period. 

 In short, the number and variety of substances and of subjects embraced under Alluvium 

 is very great, and we fancy that many persons will be surprised to learn how many 

 agencies have been at work on the surface of Vermont at a comparatively recent date, 

 geologically speaking. 



What we include under Alluvium it has been usual to regard as two formations, the 

 lowest of which is called Drift, and those deposits above it Alluvium. But since we 

 regard the whole series to be the result of the same general causes with modified action, 

 we cannot but consider the whole as a single formation. The lowest part, indeed, we call 

 Drift, and the superimposed deposits Modified Drift. In treating of them we think we 

 can give a clearer and more instructive view of their nature and origin, according to our 

 notions, by commencing with Drift and ascending to the most recent Alluvium. 



We embrace a description of the whole series under the term Surface Geology, which 

 term has indeed only recently been introduced into geology, but seems greatly needed to 

 embrace all those geological changes which the earth's surface has experienced since the 

 tertiary period ; indeed, as to one point, that of erosions, we would make it reach much 

 farther back. 



