192 



Address of Algernon S. Roberts. 



Vol. XII. 



the surface extremely rugged, and in many 

 places barren, were it not that the diluvium 

 still to a considerable extent covers the sur- 

 face, though greatly modified by the disinte- 

 gration of the rocks. 



The primitive form.ation of eastern Penn- 

 sylvania is with few exceptions stratified, 

 being the metamorphic, or changed rocks of 

 Lyell, and to this circumstance together with 

 their diluvial covering, we owe the gradu- 

 ally undulating surface, that enables the 

 farmer to work with facility. 



The soil, as before observed, is influenced 

 by the underlying rock, wherever the disin- 

 tegrating hand of time has been at work. 

 Where gneiss or laminated granite prevails 

 the soil is light and open, but not so much 

 so as where micacious stones form the sub- 

 soil or approach the surface ; while the de- 

 composition of hornblendic and argillaceous 

 rocks forms a more retentive soil, and capa- 

 ble of producing heavier crops. The happy 

 blending of these materials has, however, 

 in most cases, formed a surface soil well 

 suited to the cultivation of cereals, and of 

 natural and artificial grasses. 



The primitive formation makes its appear- 

 ance boldly in the granitic gneiss of Fair- 

 mount, but as we recede from the banks of 

 the Schuylkill, the rocks are not visible on 

 the surface until we progress further west. 

 This formation extends across the State in a 

 north-east and south-west direction, and in- 

 cludes the crystaline marble and limestone, 

 which form a still more fertile soil, adding 

 largely to the resources, both agricultural 

 and mineral, of eastern Pennsylvania. The 

 limestone is overlaid and succeeded by the 

 red and grey sandstone, and shales or middle 

 secondary formation of Rogers. 



This Is a highly valuable section for the 

 farmer, and with the addition of lime as a 

 manure becomes exceedingly fertile. This 

 secondary zone, like all the other geological 

 strata of our State, runs in a north-eastern 

 and south-western direction, crossing the 

 Schuylkill a little below Norristown, and 

 extends with few interruptions to the base 

 of the south mountains below Reading. It 

 may be known by the deep reddish brown 

 colour imparted to the soil, and which is oc- 

 casioned by oxide of iron. 



In the primitive formation is a stratum of 

 steatite or talcose rock, which crosses the 

 Schuylkill about two miles above Manayunk, 

 from which is obtained the soap-stone so ex- 

 tensively used as hearth and fire-stones, &c. 

 Where this description of rock forms the 

 subsoil or has but little coverhig, sterility 

 prevails, and it frequently makes itself 

 known by a barren strip of land. This 

 stratum is well defined where it crosses the 



Lancaster turnpike twelve or fourteen miles 

 from Philadelphia, and again near the inter- 

 section of the Columbia and West Chester 

 rail roads. The sterile character of this 

 rock is probably owing to its indestructable 

 nature, and its destitution of fertilizing ma- 

 terials. It abounds in the vicinity of Balti- 

 more, and bids stubborn defiance to the cul- 

 tivator. There, however, a variety of it 

 has been turned to profit in the manufacture 

 of epsom salts. 



In the year 1826, Dr. G. Troost, under the 

 auspices of this Society, made a geological 

 survey of the semi-circle of fifteen miles 

 around Philadelphia, limiting his examina- 

 tions to this side of the Delaware. As a 

 pioneer, he opened the way and imparted 

 considerable information for that early age 

 of the science, but his report and map should 

 be considered only as introducing the work 

 of exploration, which I regret to say, has 

 not since been followed up sufficiently to 

 make us well acquainted with the earth we 

 daily tread upon. 



Dr. Troost divides the land within fifteen 

 miles of the city, and on the western side of 

 the Delaware, into four classes, namely, 

 " 1st, granitic soils, which probably owe 

 their origin to the disintegration of gneiss, 

 mica slate, and feldspar : these soils are in 

 general, light and of a sandy nature. 



"'2nd. Amphibolic or hornblendic soils, 

 originating from the decomposition of am- 

 phibolic rocks," (and he might have added, 

 from all rocks of an argillaceous base.) 

 "These soils are of a loamy nature, having 

 generally a ferruginous colour, approaching 

 even to a brownish red, and containing a 

 large quantity of iron. 



" 3rd. Limestone soil, being partially pro- 

 duced by the disintegration of limestone; 

 and 



"4th. Alluvial soil," which includes dilu- 

 vial, no distinction being made between these 

 two classes in his report. 



Peter A. Browne, Esq., has also written an 

 essay on the diluvial formation of Philadel- 

 phia county, correcting some of the views of 

 Dr. Troost, and noticing the remarkable fact, 

 that in several — probably in all — parts where 

 the diluvial strata of Philadelphia have been 

 penetrated through, they are found to be un- 

 derlaid by an alluvium, in which are depos- 

 ited bark, shells, timber, and various organic 

 substances, proving that this formation must 

 have been anterior to the diluvial. 



Professor Rogers, in his geological survey 

 of the State of Pennsylvania, has described 

 in general terms the formations of this part 

 of the State, but almost unaccompanied 

 with agricultural views or opinions; and in 

 fact no writer on the subject with whose 



