328 The Mecroscope. 



while beneath all are the primitive formations, gneiss and granite, 

 which here form the bed of James River from Mayo's Bridge up. 

 The richest, i. e., the most purely diatomaceous part of the deposit, 

 lies somewhat below the middle of the stratum and is not more than 

 two or three feet thick; above and below a gradually increasing 

 admixture of sand and clay finally obliterates the diatoms. On one 

 slide of the smaller forms taken from the richest layer, about 70 dis- 

 tinct species have been noted, and the whole number probably largely 

 exceeds Ehrenberg's calculation. In all the other Virginia diatom 

 fields which I have examined, except the one at Petersburg, these 

 forms appear to have been much injured by chemical action, and the 

 number of species is smaller. A noteworthy fact is, that while the 

 elevation of the coast line has been so extensive and gradual as to 

 preserve the conformity of the strata (the diatoms now lying as they 

 were originally deposited), the whole stratum is traversed by seams 

 of a slate-like iron deposit from one-eighth inch to two or three 

 inches in thickness at various angles to the plane of cleavage, as if 

 some magnetic force had concentrated the iron which these organ- 

 isms are known to secrete, to the total exclusion of the diatoms. 

 Towards the edges of the field, where the deposit thins out from its 

 ordinary thickness of twenty or thiry feet to a few inches, the 

 diatoms are again lost. Repeated examinations of many specimens 

 of earth from these points, having every characteristic of the diatom 

 earth except its low specific gravity, have uniformly failed to show 

 anything but fine sand and clay. 



The City of Richmond is bisected by the valley of Shockoe 

 Creek, about one- fourth of a mile wide, and it is in this valley, on 

 the flanks of the hills and in the ravines making into it, that we find 

 the' infusorial deposit which occupies a plane at the city about ten 

 or fifteen feet above the stream. Beyond the corporation to the 

 north, a branch of the creek cuts through the stratum for half a mile. 

 While the general character of the deposit is the same, certain varia- 

 tions in the species shown on opposite sides of this valley lead to 

 the inference that the principal features of the land were shaped to 

 some extent before it emerged from the water. As before stated, 

 the characteristic forms of the Richmond diatoms are the same all 

 over the field; but the relative proportions of the species vary in every 

 locality. At a point in the Howard's Grove Valley T. Marylandica 

 is so abundant that over one hundred specimens can be found on 

 one slide; the same abundance of this form is shown in specimens 

 from the Eighth Street Tunnel, but not elsewhere. A little way 



