1885.] "•"* grazer. 



origin is of later date than the last mentioned, but it is generally used for 

 those before the date of any historical evidences of the appearance of man 

 on the planet. It is not known to me that there is a representative of this 

 age present : that marked "marl" in the geological map being intro- 

 duced without the evidence of fossils so far as I know and with consider- 

 able doubt. 



The Quaternary and Recent deposits comprise those deposits which have 

 been made from the earliest appearance of man on the planet down to the 

 present time, including of course those of origin so late that they might 

 have been historical. Such are the marks of the denudation which has 

 shaped the meadows and hills as they are at present ; the moulding of 

 the ravines and deepening of the stream-beds ; the distribution along the 

 latter of gravels, &c ; and finally (for the sake of saving one more divi- 

 sion of time, which would otherwise lie wholly within this one, and at 

 best remain very uncertain as to exact date) the works of man's hand, 

 which are discoverable in the arrow-heads and sculptures not infrequently 

 observed along the lower course of the noble river which forms York's 

 north-eastern boundary. 



One word more is necessary as to the subdivision of the rocks of these 

 different geological ages before their occurrence in York county becomes 

 our theme. 



It has been said that if the average thickness of all the strata which 

 have been yet recognized as distinct in the State of Pennsylvania were 

 laid one upon the other, the height of the pile would reach something 

 like forty thousand feet. But this is made up almost without taking 

 into account other than the Palaeozoic rocks. If the ordinary methods of 

 calculation were pursued in estimating the thickness of the Mesozoic or 

 New Red sandstone and shale alone which crosses York county, three 

 miles and a half would be added to this column.* No very great thick- 

 ness of Tertiary or Cainozoic rocks is to be found in Pennsylvania, but if, 

 instead of counting upwards, or from the most recent of the Eozoic series, 

 we were able to count downwards to its lowest member ; or to the earliest 

 existing rocks of the globe, it is probable that a thickness of this series 

 alone greater than all of those that we now know put together would be 

 established. That the exposures of rock in York county will not justify 

 the belief that any considerable fraction of this Archaean series can be 

 reached by boring, the following list of its divisions, accepted by many 

 geologists, will sufficiently show. They are given in descending order, 

 the lowest being the earliest known, and the first named the most recent : 



VI. Keweenian.f III. Huronian. 



V. Taconian. II. Norian. 



IV. Mont Alban. I. Laurentian. 



♦There are, however, good reasons for rejecting such an estimate, 

 t See volume E. p. 211, Publications of the 2d Geological (Survey of Pennsyl- 

 vania, by Dr. T. Sterry Hunt. 



PROC. AMER. PHILOS. SOC XXIII. 123. 2x. PRINTED JUNE 10, 1886. 



