OGLE COUNTY. 113 



quarry is extensively worked for mouths at a time, and carefully exam- 

 ined during all its working, fossils worth gathering may be found : but 

 a visit of a few hours to outcrops little worked at the time, cannot be 

 very satisfactory so far as the acquisition of fossils is concerned. 



The Blue Limostnne. This, the Blue limestone of the western geolo- 

 -, or the Trenton limestone of the Xew York survey, is. under pres- 

 ent classification, the Blue or Middle Division of the Trenton proper. 

 In a descending order it next succeeds the Magnesiau beds of the Galena 

 division. It is variable in appearance. The upper parts of its outcrops 

 are thin-bedded, almost shaly. and of a buff or lead-white color, often 

 breaking into small fragments when quarried. The lower layers are 

 compact and thick enough to make a good building stone. They break 

 with a glassy fracture : and some of the layers near the bottom are of 

 a deep ultra-marine blue color. This fine color fades a shade or two 

 lighter when the stones have been quarried and exposed to the weather. 



In the region of country underlaid by this rock, pit-holes, or sink-holes, 

 are of frequent occurrence. These curious depressions in the face of 

 the country are from one to three rods in diameter, and run to a 

 point in a funnel-shape, at a depth of. from six to fifteen or twenty feet. 

 The rock also contains vertical crevices, through which subterranean 

 streams of water often rush after heavy rains or springy thaws. 



Along Buffalo creek west of Polo, for three or four miles, there is an 

 upheaval of the Blue limestone. The top of the first quarry, the one on 

 the Mt. Carroll road, as already stated, is composed of Galena lime- 

 stone, shading down into beds of passage into the underlying division ; 

 but the bottom is the genuine blue "glass rock" of the Trenton. Two 

 miles below this, on the creek, several other quarries are opened and 

 heavily worked. They, and in fact all worked exposures of this rock 

 examined in this county, show substantially the following section: 



Chocolate-colored clays and subsoils, with fragments of rock and some gravel 5 feet. 



Thin-bedded, buff-colored, fragmentary limestone, sometime light lead-colored 14 " 



Heavy -bedded, blue, glassy layers, breaking with cloudy, conchoidal fracture 6 " 



These Polo quarries are worked to a depth of about twenty-five feet. 

 The blue layers in the bottom are sometimes a foot thick. When lifted 

 from their watery bed. they look as if dyed in blue ink. A large public 

 school house is now building in Polo from stone obtained at this 

 locality. The blue color is conspicuous, and the effect striking and 

 beautiful. 



This limestone also outcrops about Brookville and west of Foreston a 

 short distance, where it is quarried on some of the small feeding streams 

 of Elkhoru creek. 



On the map of Ogle county I have marked, in colors, several long 

 narrow strips on either side of Bock river. They extend diagonally 



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