90 Sir Charles Lyell [April 27, 



the line which would connect the southern train, No. 5, with its 

 supposed starting point e, is very great. One of these, 24 feet long, 

 is poised upon another which is about 19 feet in length. The 

 largest of all, composed like the rest of the green Canaan rock, lies 

 on the west flank of Dupey's Mount, and is called " the Alderman." 

 Dr. Keid measured it, and ascertained that it is above 90 feet in 

 diameter, and not much under 300 feet in circumference. At some 

 points about 40 or 50 blocks, the smallest of them larger than a 

 camel, may be seen at once. Among the larger masses the best 

 known, in consequence of its proximity to the Richmond Meeting- 

 house, belongs to train No. 6, and is that marked n on the plan, 

 Fig. 1. According to the measurement of Messrs. Hall and Lyell it 

 is 52 feet long, by 40 in width, its height above the drift in which it 

 is partially buried being 15 feet. At the distance of several yards 

 occurs a smaller block, three or four feet in height, 20 feet long, and 

 14 broad, composed of the same compact chloritic rock, and evidently 

 a detached fragment from the bigger mass, to the lower and angular 

 part of which it would fit on exactly. This erratic {n) has a regu- 

 larly rounded top, worn and smoothed like the roches moutonnees 

 before mentioned, but no part of the attrition can have occurred 

 since it left its parent rock, the angles of the lower portion being 

 quite sharp and unblunted. 



After the two great trains, Nos. 5 and 6, have crossed the ridge 

 B, and entered the Richmond valley, which is about four miles 

 broad, and about 800 feet deep below the crests of A and B, each 

 train is exceedingly well defined. They are about half a mile 

 apart, the train No. 6 varying in width from 100 to 300 feet, the 

 space intervening between them usually very free from erratics, but 

 here and there a solitary large straggler being visible. At one 

 point p, Fig. 1 , part of the train No. 6 diverges and forms a branch 

 uniting with No. 5. 



The average size of the blocks of all the seven trains laid down 

 on the plan lessens sensibly in proportion as we recede from their 

 point of departure, yet not with any regularity, a huge block recur- 

 ring here and there in the midst of a train of smaller ones. Many 

 which have wandered farthest from their parent rock retain their 

 angles extremely fresh and sharp. Almost everywhere beneath 

 the trains is a deposit of sand, nmd, gravel, and stones, for the 

 most part unstratified, and resembling the " northern drift " of 

 Great Britain and parts of the north of Europe. It varies in 

 thickness from 10 to 50 feet, being of greatest depth in the valleys. 

 The uppermost portion is occasionally, though rarely, stratified ; and 

 where stratification occurs, it seems as if the mass first thrown down 

 had been acted upon by currents, and partially rearranged. This 

 drift has been well exposed in some recent railway cuttings, where 

 it is occasionally seen to be 20 or 30 feet thick, immediately under 

 several of the trains before alluded to. The stones in general are 

 more rounded than the erraticsal ready described ; occasionally some 



