Lahee — Late Paleozoic Glaciation. 317 



(b) May contain intercalated "nests" and layers which are 

 often contorted. 



2. The matrix consists usually of rock-flour (sometimes of 

 sand). 



3. The included fragments, 



(a) If of compacted (frozen?), but not consolidated, mate- 

 rials (clay or sand) when handled by the ice, 



(1) Are usually angular, and 



(2) May show crumpling, caused by the pressure of 

 superincumbent or moving ice. 



(b) If of consolidated rock when handled by the ice, 



(1) Are generally angular or subangular, showing the 

 effects of bevelling on one or more sides and of blunt- 

 ing on one or more corners. 



(2) Are often striated. 



(3) May show concave fractures. 



B. Criteria observed in aqueo-glacial deposits. 



1. Isolated pebbles and bowlders, striated or not, may be 

 found in beds which, by their tine texture, uniform bedding 

 lamination, or lack of marked cross-bedding, indicate that 

 they were deposited by relatively weak currents or in quiet 

 water. Such pebbles and bowlders suggest ice-rafting. 



2. Contemporaneous deformation, due to grounding of float- 

 ing ice blocks.* 



C. Criteria observed in connection with the older formations 

 over which the ice may have passed. 



1. Contortion and disruption of mud and sand beds, etc., 

 suggesting that these were unconsolidated or too weak to 

 resist the ice pressure. This phenomenon is generally 

 accompanied by criteria cited under A, 3, (a). In this case 

 the surface of unconformity above the older formation is 

 very irregular and shows no striation or other evidence of 

 strong abrasion of this kind. 



2. The surface of unconformity is polished, striated, or 

 grooved, and shows other characters typical of ice erosive 

 work. These features suggest that the older formation 

 was consolidated hard rock when the ice passed over it. 



Of the criteria noted above, the only ones which are rare or 

 absent in the case of the Squantum tillite are the striated peb- 

 bles and the striated subjacent rock floor. 



For those who would question the action of ice in the deposi- 

 tion of part of the Boston Basin sediments, the following 

 remarks may be added to what Mr. Sayles has already said : — 



(1) Absence of a striated pavement beneath the tillite is to be 

 compared with the relations between the Kansan till and the 



* See Lahee, F. H., Contemporaneous Deformation : a Criterion for Aqueo- 

 glacial Sedimentation, Journal of Geology, vol. xxii. 



