>:y 



Trenton Limestone of Minneapolis and St. Paul — Hall. 118 



quality of glass could be manufactured from it. The most serious 

 difficulty to overcome will undoubtedly be found to lie in the 

 rounded form of the grains, this making the fusion of the material 

 a work of much difficulty. 



The purity of this sandstone has been alluded to. Some ex- 

 aminations by the writer show its contents of silica to be from 96 

 per cent, to 98.50 per cent., the kaolinic constituent mentioned be- 

 ing the greater portion of the difference between the numbers 

 given and 100 per cent. The parties who a few months ago 

 became interested in the establishment of a glass manufac- 

 tory in this city had a number of determinations of iron 

 oxide made by Professor Dodge, of the University of Minnesota, 

 whose several determinations averaged a littlp over 17 hundredths 

 of one per cent., an extremely low proportion for a bed which 

 undoubtedly underlies hundreds of square miles of the state, and 

 is everywhere so uniform in texture and apparently equally so in 

 chemical composition. The professor made no further determination 

 of the composition of this sandstone than is specified above . The 

 upper portion of this formation possesses a green color 

 which is quite persistent over large areas. This is perhaps due to 

 the presence of protoxide of iron coming from the formation above 

 and deprived of an opportunity to oxidize to sesquioxide. In 

 places however the almost uniform friability is lost; when the 

 waters bearing carbonate of lime, trickle down over the edges of 

 the bluffs or through fractures into the sandstone, a calcium 

 carbonate cement solidifies the mass into a very compact rock. 



There is no chemical analysis of this green intermediate layer 

 at hand, by which to compare it with the white sandstone lying 

 below it. 



The Trenton Limestone Beds. — Resting directly upon this 

 green and somewhat shaly sandstone is the bottom layer of the 

 ''Lower Trenton." It shows some interesting features. For some 

 inches up from the bottom a blue-green-gray finely textured rock 

 appears which lacks adhesion to such an extent that it very easily 

 crumbles under the hammer and for economic purposes is worth- 

 less. While the contact of this modification with the green 

 upper layer of the sandstone is quite sharply defined, its relation to 

 the limestone above and of which it stratigraphically seems to 

 form a part is not so easy to make out. If it were of the same 

 thickness and presented the same lithologic characters over a con- 



