WAVERLY GROUP. 99 



made to him by well-borers, who have rarely much knowledge of 

 lithology, while each calls the rocks by his own names, which 

 are not always easily understood. 



By the fossils contained in the rock beds we can sometimes ascer- 

 tain their horizon, but the fossils are so unequally distributed that 

 we may, in a seam which in one place contains tens of thousands, 

 fail to find a single one in another not far removed ; and where 

 outcrops are generally of so very limited extent, we often have 

 the bad luck to hit upon just such a barren portion, leaving us 

 without a key by which to ascertain the exact position of one 

 bed in relation to others. 



The thickness of the Waverly group in Michigan is considera- 

 ble, probably never less than 1000 feet, and in some places more 

 than that. Its thickness seems to be greater in the northern and 

 central parts of the peninsula than in its southern part. The upper 

 division, prevalently a sand rock, with only subordinate layers of 

 shale, and seams of harder sand rock cemented by much calca- 

 reous matter, has an approximate thickness of from 300 to 350 

 feet. Fossils are more abundant in the lower beds than in the 

 upper, and are locally distributed. The great preponderance of 

 Lamellibranches is remarkable, but I do not take this to be a 

 peculiarity of the fauna during that period, being inclined to 

 consider the nature of the deposits in connection with the animal 

 forms inclosed by them. Lamellibranches are inhabitants of the 

 sands of shore lines, and consequently must preponderate over 

 dwellers in the deep sea, which by chance only are thrown out 

 amongst them, just as is the case on the shell beaches of the 

 ocean of the present day. 



The lower division, chiefly a shale formation, is much greater 

 than the upper ; it is interstratified with arenaceous beds and with 

 seams of calcareous and ferruginous concretions ; frequently, also, 

 several heavy sand-rock masses, sometimes 100 feet in thickness, are 

 found interstratified between the shales, as is proved by deep bor- 

 ings. Whether these heavy sand-rock deposits occupy a certain 

 equivalent position in different localities, or are local deposits, can 

 not be positively asserted. All we know of the matter is derived 

 from the results of a few borings. If we compare the boring records 

 of the salt wells in different parts of the country, we find that, after 

 penetration of the upper sand-rock division, usually not less than 400 



