104 GEOLOGY OF OHIO. 



known as the " Millstone Grit," from the fact that its most compact por- 

 tions have been used for the manufacture of millstones. In our own 

 country the greatest development of this stratum is in the central parts 

 of Pennsylvania and Virginia, where it attains a thickness of from 1,000 

 to 1,400 feet. East of the Mississippi it is found underlying the Coal 

 Measures over the greater part of our coal fields, but varying much in 

 thickness. In Ohio, where present, it ranges from 10 to 175 feet ; in 

 Kentucky, from 50 to 500 feet ; in Indiana, from 50 to 100 feet ; in Mich- 

 igan, from 30 to 105 feet, etc. Thus we see that this peculiar rock is 

 very widely spread alike over the calcareous sediments and the mechan- 

 ical shore deposits of the Carboniferous sea. It occupies an area of not 

 less than 200,000 square miles in our country, and from its occurrence in 

 the same relative position on both sides of the Atlantic, apparently 

 marks a change in the physical conditions of a large part of the northern 

 hemisphere. We shall look in vain through the entire geological series 

 for another stratum of rock so widely distributed, and presenting as 

 strongly marked lithological characters as this. The coarseness of the 

 material of which it is composed, and the uniformity and wide extent of 

 its distribution, offer problems of no little interest and difficulty. The 

 pebbles it contains are generally of quartz, but not exclusively so, for 

 among them are found representatives of various other metamorphic and 

 igneous rocks, but none that are not of a peculiarly hard, tough and re- 

 sistant character. These pebbles are all well rounded, and bear evidence 

 of a great amount of trituration.* 



The sand which forms the paste that holds together the pebbles of the 

 Conglomerate is generally coarse, and consists of angular or rounded 

 grains of quartz, which differ from the pebbles only in size. 



In attempting to analyze the process by which this material accumu- 

 lated where we find it, we have to consider, first, its source, and second, 

 the mechanical agencies by which it was distributed. In seeking for 

 the source of the material we are compelled to look to such portions of 

 our continent as were, during this period of geological history, raised 



*' In the north-west corner of Holmes county the Conglomerate is thin, and irregu- 

 larly deposited, but it contains, mingled with its quartz pebbles, rather rudely rounded 

 masses of chert, generally from one to three inches in diameter, which contain Lower 

 Carboniferous limestone fossils. This would seem to indicate that the Maxville limestone 

 once reached nearly to the northern margin of our coal basin, but that the agency 

 which transported and deposited the materials of the Conglomerate had, in the 

 northern counties, broken it up and dissipated the greater part of it. The same 

 causes have also severed the connection of the limestone areas, and have given the 

 deposit the " patchy " character which it exhibits in southern Ohio. 



