GEOLOGICAL STRUCTURE. II 



became partially transformed into well-stratified layers by the same 

 floods which deposited the superimposed drift beds, whereby they 

 became blended to such a degree that a distinction of the older 

 and younger drift layers by lithological characters is impossible, 

 and can only locally be made by observation of their direct super- 

 position. The non-stratified condition of the drift is not confined 

 to the lowest beds, but repeats itself in higher positions. The at- 

 mospheric influences acting on the loose, non-indurated drift mate- 

 rial, instead of sifting and assorting the masses, as sometimes they 

 do, often, have the contrary effect ; the stratified condition of the 

 superficial masses is obscured by them, and by sliding and other 

 similar causes, they become intermingled. But, leaving this altera- 

 tion of the surface crust out of consideration, we find extensive 

 accumulations of unstratified boulder drift high above other non- 

 stratified deposits, and separated from them by well-stratified 

 layers of sand and gravel. The coarse boulder drift, covering large 

 surfaces of Washtenaw County and of other counties in the 

 southern part of the State, occupies this higher position, and 

 deeply below it, separated by intervening, well-stratified layers, we 

 find other non-stratified masses again. In the 'high bluffs on 

 Huron River, a half mile south- of Ann Arbor, an instance of this 

 superposition can be directly observed. At the foot of the bluff we 

 can see about 60 feet of a dark blue, indurated, sandy clay, mix- 

 ed with many smaller pebbles and with a few large boulders, which 

 all bear very distinct marks of glacier striation ; the clay has not 

 the least sign of gradual deposit lines ; it gives the impression of 

 a solidified mud-stream. Above it we find well-stratified sand layers 

 with discordant bedding and wedge-like intercalations of seams 

 of gravel, amounting in all to about 30 or 40 feet. The top of 

 the hill rises at least 60 or 80 feet higher, and all of it is 

 formed by non-stratified boulder drift, mixed with clay and sand 

 in such proportion as to compose a productive soil on these stony 

 hillsides. The boulders of this horizon are partly large granitic 

 blocks, or green stones, quartz conglomerates, partly blocks of the 

 Trenton, Niagara, Helderberg, or Hamilton group, with charac- 

 teristic fossils inclosed ; also, sand-rock slabs of the W'averly group 

 and from the coal measures are mingled with the others, all bear- 

 ing distinct glacial striae on their surface, but otherwise of sub- 

 angular outlines, indicating little wearing by trituration under 



