114 BULLETIN OF THE ESSEX INSTITUTE. 



leached, they may be returned to the pits whence they 

 were taken. By carefully working over the field so as 

 not to have more than a few pits open at one time, the 

 whole area might in the course of a few years be replen- 

 ished with lime carbonate at a small loss of acreage ex- 

 empted from cultivation by the pits and gravel heaps. 



A rare occurrence of an analogous series of changes is 

 the deposition of green carbonate of copper on pebbles, 

 the copper having come from the breaking up of sulphides 

 of that mineral in the overlying pebble layer. 



The iron-bearing rocks and particularly those which 

 carry both iron and lime, as in the case of the basaltic 

 rocks and the diabases of the region, have frequently un- 

 dergone decomposition to the point of losing their identity. 

 The rusty pebbles feel light or have partly fallen to pieces 

 regardless of their joint planes. In extreme cases, noth- 

 ing is left of the contour of one of these pebbles but the 

 network of quartz veins which it contained. 



The segreo^ation of oxides of iron in the outer crust of 

 diabase pebbles sometimes takes place. This crust be- 

 comes heavy and limonitic, with a bluish black tarnish. 

 A further stage in this line of alteration shows a yellowish 

 powdery center surrounded by a dark l)rown crust, trav- 

 ersed in ever}' direction by irregular wandering cracks 

 gaping at the surface and dying out inwardly, the greater 

 fractures only intersecting the nucleal portion of the peb- 

 ble. These cracks are undoubtedly due to expansion con- 

 sequent upon the oxidation and hydration of the iron in 

 the interior of the pebble. Such pebbles exposed to the 

 air and frost speedily crumble into dust. 



Owing to the low stand of the sand-plains, their bot- 

 toms generally lying at or below base-level, the streams 

 have not cut down near them so as to expose their floor, 

 and so only here and there do we see signs of accumula- 



