As a modifying -agent, it is necessary to keep in 

 mind the many small areas of lime rock, which lie so 

 scattered as to make enumeration difficult. Details are 

 given in Crosby's map of eastern Massachusetts. 



Opinions may differ as to the precise mode of origin of 

 the materials composing the drift, but there can be no 

 doubt of the main fact that they represent the rocks with 

 which we are familiar in New England, mostly consisting 

 of mica schist, gneiss, and the like. In these rocks feld- 

 spars abound, containing much potash, soda and lime, — 

 materials which become of use when decay has reduced 

 the rock to the condition of mud or clay. 



It is not probable that the decay of these rocks in situ 

 has contributed much to the soil of the region. They 

 are mostly durable, and it is exceptional to find them 

 decayed to any great depth. The period of time, too, 

 during which such decay is conceivable, is a relatively 

 short one — since modern observers incline to the view 

 that only from 6000 to 10,000 years have elapsed since 

 the glacial period. 



The diversity of physical conditions gives rise to a cor- 

 responding variety in the character of the flora. Poten- 

 tilla tridentata, Vacciniunn Canadense, Ribes prostra- 

 tum, Acer spicatum, Abies balsamea, Taxus baccata, 

 var. Canadensis, and Dalibarda repens have been found 

 indisputably native only upon or near Mt. Watatic ; Viola 

 rotundifolia in a deep ravine at Ashby ; Dirca palustris 

 and Lonicera ccerulea, at Townsend ; Alpine lichens and 

 mosses are occasional upon high hills ; while Ledum lati- 

 folium, Kalmia glauca, Andromedia polifolia, Chio- 

 genes and Smilacina trifolia linger here and there in cold 

 sphagnum swamps. 



A few more southern species are sometimes met with, 

 among which are Draba Caroliniana, Woburn ; Draba 



