316 Lahee — Late Paleozoic Glaciation. 



Art. XXY. — Late Paleozoic Glaciation in the Boston Basin y 

 Massachusetts ; by Frederic H. Lahee. 



The Boston Basin is essentially a down-faulted block of 

 sedimentary rocks and associated lavas which were folded and 

 locally metamorphosed at the close of the Paleozoic Era. They 

 are thought to be of late Carboniferous or of Permian age. 

 As regards the stratigraphic sequence, the "Roxbury conglom- 

 erate" constitutes the lower 3000 feet,* and the "Somerville 

 slates" constitute the upper 2300 feet,* of the formation. 

 Where these two groups of strata pass into one another there 

 is a considerable thickness (two or three hundred feet) of tran- 

 sition beds — shales or slates, sandstones, and conglomeratic rocks. 

 The main mass of the Roxbury conglomerate displays charac- 

 ters suggestive of piedmont fluviatile deposition, but within 

 the transition beds are several features which point strongly 

 to Paleozoic glaciation in this vicinity. 



In his "Squantum Tillite"f, Mr. R. W. Sayles has ably pre- 

 sented the case for glacial action in the deposition of the upper 

 part of the Roxbury conglomerate. It is this upper portion, 

 belonging to the transition beds, that he calls the Squantum 

 tillite. While Mr. Sayles' paper is written with a force and 

 clearness which should convince, nevertheless there may still 

 be some of his readers who will remain somewhat skeptical. 

 Having some familiarity • with the Boston Basin and, at least 

 in the large, strongly favoring Mr. Sayles' convictions, I there- 

 fore take this opportunity of offering a few suggestions. 



Has not Mr. Sayles used the word criterion inadvisedly ? On 

 page 144 of his article he lists fifteen characteristics of the 

 Squantum tillite and subsequently refers to these as criteria. 

 The presence of "Some rounded, water-worn pebbles or bould- 

 ers" is not a criterion for the Squantum tillite nor for any other 

 tillite or till. They do not assist in the determination of a 

 deposit as till or tillite. The same may be said of a few of the 

 other phenomena cited in the list. 



Criteria which indicate the action or association of ice in the 

 deposition of any sedimentary series may be classified as fol- 

 lows : — 



A. Criteria observed in ice-laid deposits (till and tillite)4 



1. The deposit as a whole, 



(a) Is "an unstratified mass of miscellaneous and unsorted 

 rock materials." 



* Mansfield, G. E. : The Origin of the Eoxbury Conglomerate. Mus. 

 Comp. Zool., Bull., xlix, pp. 209-210, 1906. 



fMus. Comp. Zool., Bull., lvi, pp. 141-175, 1914. 



|In this classification I have drawn freely from Mr. Sayles' paper. 



