254 GEOLOGY OF EASTERN WISCONSIN. 



liers, while the matrix is of white sand similar to that of the St. 

 Peters sandstone. There are also fragments of sandstone contain- 

 ing the cylindrical cavities known as Scolithus. A single bowlder 

 was also found uniting the two. I have found Scholithus tubes in 

 the upper transition layers of the St. Peters sandstone, and as there is 

 no known Potsdam sandstone along the line of drift for more than 

 one hundred miles, and rarely then, in contact with quartzite, it seems 

 altogether most rational to conclude that the St. Peters sandstone 

 was laid down around the island of quartzite from which the pebbles 



FIG. 21. 



* 80 Hotfs 



PROFILE SECTION SHOWING THE RELATIONS OF THE WATERLOO QUARTZITE. 

 1. Quartzite. 2. St. Peters Sandstone. 3. Trenton Limestone. 



of the conglomerate were derived by beach action, and that the 

 bowlders in question were derived from the deposit thus formed. The 

 greater fineness of the rock, which is a matter of observation, may 

 account for the preservation of the Scolithus tubes, which are very 

 rare in the more friable portions. This view is both corroborated and 

 complicated by the still more interesting facts observed in the town of 

 Portland. Opposite Rock Island and near the water's edge, we find a 

 white sandstone bearing abundant well preserved Scolithus tubes. 

 This graduates above into a fine conglomerate, which becomes coarser 

 and coarser until at a height of 48 feet, where it adjoins the quartzite, 

 it consists of bowlders three or four feet in diameter, imbedded in. 

 finer grades of conglomerate. There is here no question as to the ori- 

 gin of the conglomerate, or of the relations of the Scolithus to it. 

 The quartzite rose as a rocky island in the depositing seas, and yield- 

 ed its material to the beating of the waves, by which the conglomerate 

 was formed. 



But on the opposite side of the ridge, somewhat more than a mile 

 distant, a peculiar shaly, arenaceous rock is found at the same eleva- 

 tion. One variety of this rock is exposed in the railroad cut in Sec. 

 3 of the town of Waterloo. It is of variegated, reddish aspect and 

 irregular texture, and closely resembles the variegated shales of the 

 Mendota beds of the Potsdam, and also some of the modified forms of 

 the St. Peters sandstone, where it lies contiguous to the domes of- 

 Lower Magnesian limestone, subsequently to be described. It has 

 been penetrated at several points in the vicinity by wells which in 

 some cases reach the quartzite underneath it. The owners of the 

 wells usually describe it as a red, sandy rock. At one point (middle 



