8 



2. The Huronian, just outside of this, beginning at 

 and including the Middlesex Fells, comprises primitive 

 rocks granite, petrosilex (porphyry), diorite, horn- 

 blendic gneiss, quartzite, quartzy slate. Limestone occurs 

 in patches. 



3. The Montalban (later in date than the Huronian, 

 but antecedent to the Cambrian), lies to the north and 

 west of the Huronian, and comprises granite, gneiss, mica 

 slate, argillite, and numerous patches of limestone. It 

 occupies the larger part of the county, and is bounded by 

 a N. E. to S. W. line which includes the towns of Wilming- 

 ton, Bedford, Concord, Sudbury, and Framingham. 



The soil, like that of most parts of New England, is 

 mainly dependent for its characteristics upon the glacial 

 drift, which covers most of the rocks to the depth of 

 many feet. This material consists of two portions ; the 

 very compact boulder clay or till, often called "hard 

 pan ;" and a loose mass, of gravelly and sandy consistency, 

 which has been derived from the boulder clay by the 

 the washing of ancient torrents. The latter is often 

 stratified, is comparatively free from boulders, and forms 

 the present soil, with such additions as the yearly decay 

 of vegetation for many centuries has made. Its qualities 

 vary greatly, some having been deposited in the form of 

 sand, or sterile gravel, while other parts are of a rich, 

 clayey nature. "Terraces of sand arid gravel from the 

 re-assorted boulder clay make up by far the greater part 

 of the low-lying, arable lands of eastern Massachusetts ; 

 and of this nature are about all the lands first used for 

 town-sites and tillage by the colonists, notwithstanding 

 the soil they aiford is not as rich or as enduring as the 

 soils upon the unchanged boulder clay." N. S. Shaler, 

 Memorial History of Boston, vol. 1, p. 6. 



