REPRESENTATIVES OF PRE-WISCONSIN TILL 319 



somewhat feldspathic sandy matrix. It exhibits a rather perfect 

 cleavage at right angles to its stratification. Where so situated 

 as to be exposed to the full action of the ice at the time of the 

 last invasion, it exhibits smooth, hard, and well glaciated sur- 

 faces of a dark gray color with almost no evidences even of 

 superficial oxidation. In less exposed positions, such as exist 

 along the well defined valley running southward through the city 

 of Brockton just east of the center, the glaciation was apparently 

 exceedingly slight. It is in such positions that the decayed 

 conglomerate from which the lower till was derived is found. 

 At the surface the conglomerate presents the high colors of 

 advanced oxidation, but somewhat irregularly arranged owing to 

 variations of the original composition of the rock. Some of the 

 portions free from iron give on decay spots or streaks of an 

 almost white sandy clay. The predominating color is a distinct 

 yellow, interspersed with red in many places. The rock is so 

 soft at the surface that it can sometimes be removed by pick and 

 shovel. The depth of the extreme decay is somewhat variable, 

 possibly averaging from two to three feet, though it is probably 

 considerably greater in places. From the highly decayed por- 

 tions, the rock passes downward by insensible gradations into less 

 altered portions, but in none of the shallow excavations which 

 the writer has seen has fresh rock, such as is exposed where the 

 glaciation has been severe, been reached. The decomposed con- 

 glomerate probably underlies most of the low region near the 

 center of the city and has been exposed in the laying of water 

 pipes, drains, sewers, etc., along Center and Crescent streets and 

 near the high-school building on Main street. Many of the exca- 

 vations in which decomposed conglomerate was exposed were 

 made ten or more years ago, and though the presence of the 

 decayed rock can be vouched for, the writer cannot say with cer- 

 tainty that it was everywhere overlaid by the oxidized till, 

 though later observations suggest that such was probably the 

 case. 



It is certain that the deep rock decay antedates the last ice 

 invasion. If this decay is the result of pre- Pleistocene weathering, 



