528 STUDIES FOR STUDENTS 



Further, a careful examination of the polished faces of this rock 

 shows that the feldspar crystals are broken by numerous cross 

 fractures. These small, scarcely perceptible fractures may 

 account in part for the less load which the rock is capable of 

 sustaining normal to the rift. The cone which remained after 

 crushing this sample of rhyolite is shown in Plate I. 



The highest crushing strength obtained for true granite was 

 43,973 pounds per square inch, which was obtained on a sample 

 of the Montello stone. This, so far as my knowledge goes, is 

 the highest crushing strength that has been recorded for any 

 United States granite. It exceeds the highest test on the 

 Fourche Mountain granite 1 of Arkansas by nearly 15,000 pounds 

 per square inch, while the highest test on the granite from St. 

 Cloud, 2 Minn., is 16,000 pounds less than this. The granite 

 from Redgranite, Waushara county, also gave a crushing 

 strength of over 36,000 pounds per square inch, which exceeds 

 the highest test made on the Fourche Mountain granite by 

 7000 pounds per square inch. These illustrations give evidence 

 that in at least three different areas in Wisconsin granite and 

 rhyolite occur, which, as far as known, surpass in strength gran- 

 ite or rhyolite from any other quarry in the United States. 



Most of the granite samples broke with an explosion. Ordi- 

 narily an upper pyramid or cone, such as shown in the accom- 

 panying illustration, Plate I, was all that remained after the test. 

 In a few cases a lower or opposite pyramid remained, but as a 

 rule this part of the sample was reduced to powder. In many 

 of the cones that remained a concentric structure had been 

 developed through the pressure, which had much the appear- 

 ance of cleavage. This is nicely shown in the accompanying 

 illustration, Plate II. 



Thirty-one tests were made on limestone from eleven differ- 

 ent quarries. The strongest sample tested gave a crushing 

 strength of 42,787 pounds per square inch, which is about 1 8,000 

 pounds higher than any known test recorded for limestone, 



•Annual Report Arkansas Geological Survey, 1890, Vol. II, p. 42. 



2 Geological and Natural History Survey of Minnesota, 1884, Vol. I, p. 196. 



