258 SURFACE GEOLOGY. 



Supposing the course of the stride about S. 20° E., the Concord granite 

 would have come from Farmington, 15 miles distant; the sienite from 

 Gunstock, from 25 to 35 miles ; the Montalban rocks about the same 

 distance, or a httle more. 



Manchester. 



The city authorities have made numerous cuttings on various kinds of 

 material within their limits, which can afford the most abundant informa- 

 tion respecting the different kinds of material present in the drift. I 

 searched for rocks transported a great distance rather than for the entire 

 variety, so that my list is small. On Wilson's hill, at Lowell street, exca- 

 vations have been made to the depth of from 12 to 15 feet, the finer 

 sandy and gravelly portions separated by screens and carted away, leav- 

 ing the stones and boulders, none of which exceed two or three feet in 

 diameter. The excavation shows several features concerning the nature 

 of the till better than anything seen elsewhere, and deserves to be shown 

 in photographic illustrations. The examination was made before our 

 decision to distinguish between the lower and upper till. The excava- 

 tion probably exhibits both kinds of till, the upper a coarse, yellow gravel, 

 from two to three feet thick. The surface of the hill is smooth, with 

 scarcely any large boulders visible. There is a layer several inches thick 

 of a sand overlying the gravel. Roots of pine trees have penetrated the 

 earth to the depth of six feet. The lower till here contains many glaci- 

 ated stones, including those that have come from the greatest distances. 

 The material resembles clay, but, when pulverized between the thumb 

 and fingers, is seen to be gritty, composed of siliceous rather than alumi- 

 nous particles. This remark is applicable to very much of the drift de- 

 posits of southern New Hampshire, and explains our preference for the 

 name till, adopted in this report for these accumulations, rather than 

 boulder clay. The till here presents a laminated appearance, with hori- 

 zontal partings, and iron oxide in the seams, evidently derived from water 

 soaking down from the soil above. This laminated aspect is very com- 

 mon in the lower till, and is believed to result from the enormous pres- 

 sure exerted by the presence of the ice-sheet. Two thousand or three 

 thousand feet thickness of ice are adequate to induce these cleavage 

 planes in the moist and slightly plastic earth. They cannot have come 



