[23] 



38 



ry." Here the strata assume a somewhat different character. Above, 

 they are of a crumbUng, argillaceous, marly nature, producing a retentive 

 soil", well adapted for the growth of wheat and corn ; beneath these disin- 

 tegrating beds are red and rather argillaceous freestones of an inferior 

 quahty, which pass downwards into hght greenish-gray beds, varying 

 from a foot to two or two and a half feet in thickness. 



At about fifteen feet from the top is the most important bed exposed in 

 the quarry, of a grep.nish-gray hue, usually called the " dove-colored bed;" 

 see specimen No. 21. Beneath this, as far as can be ascertained from 

 the rubbish strewed over the lower face of the quarry, are brown and 

 bluish-purple beds, (No. 22,) of inferior quality. Here, as at Bull run, the 

 strata dip to the west at an angle of about 15° or 20° ; so that the middle 

 layers crop out towards the summit of the hill. Near the top the dove- 

 colored bed is two feet thick. Sixty feet down the slope of dip it has in- 

 creased to four feet; forty-five feet more, it is six feet; and fifty-four feet 

 more, it is nine feet thick. At the same rate of increase to the west, be- 

 yond where it is exposed to near the level of the canal, it would be twelve 

 to thirteen feet. The color of this bed is rather cold, but it is much ad- 

 mired by some persons. It is faintly striped parallel to the stratification, 

 (see 21a); it has also some small faint spots, or "pock," as the stone- 

 masons call it, (see 2la), not quite so hard as the body of the rock. This 

 stratum is not near so easily worked as the best beds in Bull run quarry ; 

 indeed it is harder to work than marble. If this dove-colored bed is 

 worked extensively, there will be considerable expense incurred in strip- 

 ping, since there are fifteen feet of solid strata overlaying it. 



A few hundred yards west of this is another quarry, equally bold, and 

 composed of similar beds, their tints being for the most part gray, green- 

 ish gray, and dove-colored. 



Beyond this, along the canal, the ground is flat and wet, and the hills 

 recede towards the north, so that to the Avest of the last described quarry, 

 for fifteen or twenty miles, building-material cannot be procured so con- 

 venient to navigation. 



Between the College quarry and Bull run there are several localities 

 close to the canal, where some rock has been quarried. The beds ex- 

 posed are of warm red tints, similar to the red sandstones in Bull run. 

 At these quarries, and in Bull run, the material for the construction of the 

 aqueduct and locks in this part of the Ohio and Chesapeake canal were 

 obtained, lliese works have stood the test of time as well, if not better, 

 than any of the other structures along the whole line of this improve- 

 ment. 



The investigations in the vicinity of Seneca creek prove conclusively 

 that the blufis on the banks of the canals, for about three-quarters of a 

 mile, afford abundance of freestone for building purposes, equal and even 

 superior to those which are obtained in the quarries of New Jersey, which 

 supply the New York market. 



If this freestone should ultimately become the choice of the building 

 committee of the Smithsonian institution, a very careful selection will be 

 necessary not only as regards the particular color preferred, hut that it be 

 those softer varieties of pure grit-stone, free from all argillaceous or marly 

 admixture, and containing none of those " pock" marks, or small spots 

 and cavities, which not only injure its appearance, but detract from its 



