144 bulletin: museum of comparative zoology. 



has usually a finer matrix than the upper part, and in the lower part 

 the striated pebbles are much more abundant. 



In beds of till or tillite intercalated layers or beds of gravel, sand, 

 or clay may be found. Strictly speaking till is unstratified, but Sir 

 Archibald Geikie (1903, p. 1310) writes as follows: — "In general, 

 boulder clay is unstratified, its materials being irregularly and tumultu- 

 ously heaped together. But rude traces of bedding may not infre- 

 quently be detected, while in some cases, especially in the higher clays, 

 distinct stratification or intercalated seams of sand or gravel may be 

 observed." Prof. James Geikie (1895, p. 14) describes these intercala- 

 tions as follows: — " Another characteristic of till is the not infrequent 

 occurrence of included nests and lenticular laN^ers, and occasionally 

 thick beds of gravel, grit, sand, and brick-clay. Sometimes these lie 

 in approximately horizontal or gently inclined positions, but usually 

 they are more or less disturbed, and often curiously curled up and 

 contorted, so as to present the appearance of having been rolled over 

 upon themselves along with the clay in which they are inclosed." 



Considering all the different characteristics of till and tillite, it is 

 necessa«-y to conclude that the proof of till or tillite lies, not in the 

 finding of any one or two glacial characters which a given deposit may 

 possess, but in the sum total of the characters found. In the different 

 exposures of a bed which has the appearance of till or tillite, there 

 should be found a majority of the characteristics of till. The list of 

 criteria given above contains the most important characters, and the 

 following resume is given as an aid in forming an opinion of the glacial 

 characters found in the tillite described in this paper. 



A. An unstratified mass of miscellaneous, and unsorted rock mate- 



rials. 



B. The matrix usually of rock-flour or clay, but sometimes of sand. 



C. The included rock fragments usually angular or subangular, 



with several facets. 



D. Some rounded, water-worn pebbles or boulders. 



E. Striated pebbles, rock fragments, and boulders. 



F. Stones blunted at one or both ends, or rather pointed at one end 



and blunted at the other end. 



G. Fragments bevelled on one or more sides, the sides usually not 



parallel but making an angle with each other. 

 H. Concave fractures. 

 I. Till or tillite resting on a grooved or striated rock-floor or striated 



pavement. 

 J. Great variety in kinds of materials. 



