PRELIMINARY CHATTER. 21 



torn from fissures more hidden still, will always, and in all 

 time to come, dax/le the eve and mind, while the more 

 modest merits the rich, warm cream, color, its enduring 

 qualities as a building material, the good lime easily burned 

 from it are almost forgotten or overlooked. 



Thv Bhic Limextone. Xext succeeding in the descending 

 order comes the Blue limestone, or Trenton limestone pro- 

 per, of the earlier Western geologists. It is now regarded 

 as the middle division of the Trenton group, the Galena 

 above and the Buff below, both being now regarded as 

 members of the Trenton. The upper strata are thin-bed- 

 ded, and of an ashy-white or dirty buff color. The lower 

 strata or layers are thicker bedded, and of an intense ultra- 

 marine blue, when first quarried, but afterwards bleach out 

 to a paler or whiter blue. The whole of this division has 

 a more or less conchoidal or glassy fracture, when broken ; 

 some of the bluer strata are exceeding conchoidal in their 

 fracture, and have been characterized in the common 

 speech, all over the lead mines, as the "glass rock." The 

 Blue limestone reaches a thickness in this part of the State 

 of from forty-five to sixty feet. It makes an excellent, and 

 Avhen properly dressed and mingled in its shades of color, a 

 beautiful building material. The union school building 

 in the town of Polo, a very handsome and tasteful structure, 

 is built of this Blue limestone. An excellent article of com- 

 mon lime may be burned from it. All around the lead re- 

 i:i<>ii. and where the streams cut through the Galena lime- 

 stone, the Blue limestone appears. 



It is one of the most fossiliferous deposits in this part of 

 the State. A large species of Ortlim-crax. sometimes six or 

 eight inches in diameter, and eight or ten feet long, is often 

 found. A large shell, in a fossil state, related to the Nau- 

 tilus, perhaps the lAtmtcn undattw of Hall, is not uncom- 

 mon. Corals, trilobites. and many species of shells, and 

 some eiicrinites. are found in abundance, especially on 

 Rock river, in the neighborhood of Dixon. 



