LOWER MAGNESIAN LIMESTONE. 269 



fined to any one horizon, though most abundant in the middle por- 

 tion. Its distribution is irregular, though the nodules are frequently 

 arranged in layers along the bedding planes of the limestone. 



The more distinctly crystalline quartz forms, in some portions of 

 the deposit, multitudes of little clusters, completely filling small cav- 

 ities in the rock mass, and in other portions, where the cavities are 

 larger, the crystals only form a lining, producing drusy little grottoes, 

 some of which are very beautiful. The quartz is most frequently 

 transparent or opalescent, but it is sometimes red, brown, or rose 

 colored. The crystals are sometimes grounded on a chalcedonic base, 

 forming a beautiful combination. 



The quartzose sand is confined chiefly to the vicinity of the junc- 

 tion with the sandstone below and above, and to a subcentral band of 

 shale, subsequently described. A portion of the oolitic grains Lave a 

 silicious core. 



Argillaceous material is not abundant in the formation, except in 

 shaly bands, where it sometimes constitutes as much as 20 per cent, 

 of the whole. In the upper part of the formation it sometimes 

 amounts to six per cent., but it is seldom that the rock is notably ar- 

 gillaceous. Neither is the amount of iron conspicuous, though its 

 compounds sometimes reach four or five per cent. The rock seldom 

 appears ferruginous. 



In addition to these chemical and crystalline characters, the oolitic 

 structure distinguishes some portions. In most cases, the spherules 

 differ but little in size from those of the roe of our common fish, 

 which they so much resemble, but some, as those at Oconto Falls, 

 reach a much larger size. It is a significant fact that the oolitic struc- 

 ture is confined to essentially the same horizon with the sand above 

 mentioned. A portion of the spherules are simply grains of sand, 

 coated with concentric layers of carbonate of lime and magnesia. 



Passing from these to the more massive features, the rock presents 

 a very irregular structure, owing to unevenness of hardness and com- 

 position, and inequality of deposition. The effect of weathering is to 

 exaggerate this, and hence outliers of this formation present a very 

 rough and often grotesque exterior. A portion of the rock is brecci- 

 ated, having been apparently once broken up by the waves, and in 

 some cases somewhat rounded by rolling, and afterwards recemented 

 by material similar to the fragments themselves. These layers add 

 to the coarse aspect of the rock. In addition to this, the bedding is 

 often very irregular, and sometimes obscure, and the beds not unfre- 

 quently undergo change when traced horizontally. The color of the 

 weathered and leached portions is a dirty white, gray, or very light 



