816 



SCIENCE, 



[N. S. Vol. IV. No. 101. 



deny that the variegated Potomac clays in 

 Maryland are continuous with those in 

 Delaware, Pennsylvania, and ISTew Jersey, 

 and that the similar basal clays on Long 

 Island, and the other islands to the east- 

 ward as far as Nantucket, are part and 

 parcel of the same series. There is now 

 positive proof that the southern end of this 

 series is Jurassic, and it is certainly a fair 

 conclusion that the remainder is of the 

 same age. The burden of proof will rest 

 upon those who hold to the contrary. 



To place the strata in question in the 

 Jurassic section of the Atlantic coast at 

 once removes many difficulties that have 

 hitherto perplexed students of the Mesozoic 

 of this region. It completes the series, and 

 shows in part, at least, what was done in 

 deposition during that long interval be- 

 tween the end of Triassic and the beginning 

 of Cretaceous time, when the great barrier 

 was broken down, which, from the Devon- 

 ian to the Cretaceous, shut out the waters 

 of the Atlantic. 



I must leave it to others with leisure at 

 their command to work out the details of 

 this well-marked series, and its relation to 

 those above and below. I have no time to 

 devote to the surface geology of this belt or 

 to the eai-lier deposits of Tertiary time. 

 Just now the Mesozoic interests me most of 

 all, especially its middle section, the 

 Jurassic, as I believe great injustice has 

 been done, since this has been denied its 

 rightful place, and a name not its own 

 stamped upon it. 



In a later communication I hope to dis- 

 cuss this question further, and especially 

 the Jurassic beds south of the Potomac 

 Biver. O. C. Maesh. 



Yale University, New Haven, Conn. 



VOLCANIC ASH IN SOUTHWESTERN NE- 

 BRASKA . 

 In the summer of 1894 I spent two days 

 in southwestern Nebraska in the examina- 



tion of certain deposits of volcanic ash. 

 These deposits were seen at three localities, 

 viz : 1° near Ingham, Lincoln county ; 2° 

 near Edison, Furnas county, about forty- 

 five miles southwest of Ingham; and 3° 

 near Orleans, Harlan county, about twenty 

 miles southeast of Edison. At all of these 

 localities there are several more or less 

 closely associated exposures of the ash. In 

 all cases it appears in the side or near the 

 head of a canyon-like ravine. In the 

 ravines where it occurs it is to be seen only 

 where the walls are essentially vertical, and 

 in all places it is capped by loess. It is 

 probable that the failure of the ash to ap- 

 pear at many points on the sides of the 

 ravines in which it occurs is due to the 

 fact that the loess from above has slumped 

 down, concealing it. 



The deposits near Ingham. The volcanic 

 ash near Ingham is exposed at four or five 

 points, the most widely separated of which 

 are less than a mile apart. The first sug- 

 gestion of the exposures was that the ash 

 formed a continuous layer beneath the loess, 

 and that it failed to appear continu- 

 ously in the wall of the ravine only be- 

 cause it was locally concealed by the slump- 

 ing of the loess above it. Further examina- 

 tion, however, showed that this was probably 

 not the fact, for at one point a layer of ash 

 was seen to thin out promptly from a thick- 

 ness of twenty-two feet to a thickness of 

 four or five. 



The ash at this point varies in color from 

 nearly white to a yellow-cream color, on 

 the one hand, and to a light gray, on the 

 other. It varies in grain from the grade of 

 coarse sand to that of fine flour. These 

 grades of coarseness do not appear to be 

 the result of admixture with foreign sub- 

 stances, for no such materials could be 

 detected on the ground, and microscopic 

 examination confirmed the field conclusion. 

 The ash is more or less affected by streaks 

 or pockets of loess which appear to repre- 



