160 bulletin: museum of comparative zoology. 



of the tillite and transition-beds and the uppermost part of the slate 

 formation. The strike here is N 12° E, and the dip 70° X. The 

 tilhte exposure proper is very small and little can be said of it. The 

 matrix of the tillite is sandy. The pebbles and boulders are angular, 

 subangular, and rounded. The transition-beds are very much like 

 those at Squantum Southeast: — large boulders and slate fragments 

 mingled in an unstratified mass, with here and there thin layers of 

 sandstone. An ice-rafted boulder was extricated from the slate, and 

 many more may be seen. 



MelaphjTe appears about fifteen feet below the tillite, but whether 

 as a flow, dike, or sill has not been satisfactorily determined. 



Crosby studied this locality some years ago and wrote (1894, p. 249) 

 as follows concerning the tillite: — "The pebbles are of all sizes up 

 to a yard or more in diameter, the largest observed being a boulder 

 of coarse granite over 5 feet in length. Furthermore the various 

 sizes are jumbled together promiscuously without evident assorting 

 or stratification, looking not unlike an indurated till or boulder clay." 

 Crosby discovered some exotic limestone here, which he thinks came 

 from the north. He did not prove his conclusions, however, on 

 this point (Crosby, 1894, p. 265-266). 



Cleavage is well marked. 



Criteria found: — A, B, C, D, F, J, L, M, N, O. 



Locality. — Arnold Arboretum. "While the proof of this paper was 

 in press, I discovered a large tillite locality in the Arnold Arboretum 

 north of Peters Hill. It is the ridge covered by evergreens on the 

 eastern margin of the Arboretum grounds. This is along the same 

 strike as Locality 3, but farther northeast. No contacts with other 

 beds have been seen, so it is impossible to obtain strike and tip. 

 Criteria observed during one visit: — A, B, C, D, J, M. 



Review of localities with criteria. 



The list of localities on the following page with the criteria of tillite 

 found at each, are arranged as nearly as possible according to the 

 •extent of outcrop, and favorable conditions for search. 



The list below shows that where there is ample opportunity, abun- 

 dant criteria are usually found. It must be noted that a thorough 

 examination of some outcrops has been impracticable as yet, owing 

 to location in private grounds or cemeteries. Other outcrops are so 

 limited in extent that they show only the presence of the formation. 



