GEOLOGY. 237 



Prof. Shaler says that no perfectly satisfactory explanation has 

 vet been offered. Pie seems, however, to incline towards an 



J 



hypothesis similar to that offered by Sterry Hunt to account for 

 the massive apatite beds of the early palosozoic rocks ; namely, 

 the action of quantities of unarticulated brachiopods separating 

 phosphate of lime from the waters of the sea. In this connection 

 he says : 



" We know that some of the pteropod mollnsks, forms which 

 are frequently abundant in the ocean at great distances from the 

 land, have a composition not materially different from that of 

 bones. It has even been stated, though I do not yet know by 

 what authority, that some of the marine alga? contain a large per 

 cent, of phosphate of lime. The fact of the existence of this 

 material in a number of the inferior organizations of the sea 

 makes it, in most cases, more reasonable to account for the for- 

 mation of extensive masses of phosphate beds by the deposition 

 of the remains of invertebrate species, than to suppose that they 

 were accumulated by vertebrate animals." Proc. Boston Soc. 

 Nat. Hist. 



SURFACE GEOLOGY OF THE BASIN OF THE GREAT LAKES. 



Dr. Newberry, in a paper on the " Surface Geology of the 

 Basin of the Great Lakes and the Valley of the Mississippi,' 1 says : 



" The most important facts which the study of the drift phenom- 

 ena of this region have brought to light are briefly as follows : 



" 1st. In the northern half of this area down to the parallels 

 of 3S-40 we find not everywhere, but in most localities where 

 the nature of the underlying rock is such as to retain inscriptions 

 made upon them, the upper surface of these rocks planed, fur- 

 rowed, or excavated in a peculiar and striking manner. No one 

 who has seen glaciers, and noticed the effect they produce on the 

 rocks over which they move, will fail to pronounce these markings 

 the tracks of glaciers. Though having a general north-south 

 direction, locally the glacial furrows have very different bearings, 

 conforming in a rude way to the present topography, and follow- 

 ing the direction of the great lines of drainage. 



"2d. Some of the valleys and channels which bear the mark 

 of glacial action are excavated far below the present lakes and 

 water-courses which occupy them. These valleys form a con- 

 nected system of drainage at a lower level than the present river 

 system, and lower than could be produced without a continental 

 elevation of several hundred feet." As examples of this Dr. 

 Newberry cites among others the following : An old excavated, 

 now-filled channel, connects Lakes Erie and Huron, the rock sur- 

 face at Detroit being 130 feet below the city ; an excavated trough 

 runs south from Lake Michigan, penetrated at Bloomington, 111., 

 to the depth of 230 feet ; the rock bottoms of the troughs of the 

 Mississippi and Missouri near their junction, or below, have never 

 been reached. The borings for oil in the valleys of the western 

 rivers have afforded opportunity for demonstrating the existence 



