295 



acid. It is therefore not well adapted for the kiln. The upper, yellow^ 

 or brown clay, is the least marly, and is better suited to brick manufac- 

 ture. Tlie beds are often stratified, and porti-:>ns may be found more 

 nearly free from this injurious ingredient. 



The presence of carbonate of lime may be detected by dropping a 

 small bit of the clay into an acid, (strong vinegar will answer,) when, 

 if the clay be marly, effervescence ensues, occasioned by tlie decompo- 

 sition of the carbonate of lime, and escape of the carbonic acid, which 

 is always combined with the lime. In this way, it will often be easy to 

 distinguish such portions of a bed as do not contain lime in sufficient 

 quantity, to impair their fitness for ordinary purposes. 



Beds of clay, of a few feet thickness, often found alteinating with 

 strata of gravel and sand among the diluvium, and similar beds, occu- 

 pying acres of limited extent on the surface, are frequent in the sandy 

 soils of the openings. The latter seldom exceed a few acres in extent, 

 and generally are of much smaller dimensions, with a thickness rarely 

 exceeding four feet. 



These isolated beds of clay are almost universally free from any inju- 

 rious proportion of lime. In this particular, they are superior to the 

 clays above mentioned, for making a durable brick. These beds are, 

 however, very silicious, and indeed, partake of aU the intermediate condi- 

 tions, from a stiff clay to merely an agglutinated sand. Proper precau- 

 tions are not always observed in this particular; for notwithstanding 

 that sand often constitutes by far the largest constituent, an additional 

 supply is not uncommonly added in the process of manufacture, almost 

 to the total destruction of the adhesive property of the material. I may 

 here observe, that in general these surface beds contain, without artificial 

 mixture, sufficient sand to subserve their purpose in the manufacture of 

 bricks. 



ERRATIC, FRAGMENTARY ROCKS. 



Boulders of the primary rocks are found in great numbers, lodged 

 upon the more elevated and broken parts of the country, and imbedded 

 in the diluvial gravels. They comprise a great variety of granites, 

 quartz and hornblende. The hilly region of Ann Arbor affords a fine 

 locality for procuring a set of almost every variety found in the State. 



Large fi-agments of limestone are occasionally to be met with, which 

 have been disruptured from the transition and carboniferous limerocks 



