GEOLOGY. 119 



sia,) and then are formed the Hornblende and Syenite rocks, in whicli 

 Quartz is either wanting or exists usually in but small quantities. The 

 Syenite rock and Greenstone are abundant in Adams count}', differing 

 from each other in nothing so much as in texture, in the former the con- 

 stituent minerals being large and distinct, which is not the case in the 

 latter. Talc and Chlorite exist abundantly as materials in the compo- 

 sition of rocks. We have both talcose and chlorite slates in the axis 

 and along the south-eastern flanks of the mountain, which skirts this 

 county on the North-west. Limestone itself also forms extensive beds 

 of rocks, and is to be found in almost every part of our country. This 

 is decidedly the most valuable rock to man, both on account of its ad- 

 vantage to the soil in increasing its productiveness, and its uses for econ- 

 omical purposes. The slates are chiefly composed of sand and clay, 

 resulting from the pulverized materials of lower rocks, and although ex- 

 tensively distributed, seem, in general, to be chiefly useful in aflbrding 

 the materials for fresh soils. 



2. A second circumstance, calculated to arrest our attention in an ex- 

 amination of the structure of the globe, is that the higher rocks or those 

 which are nearest the surface are generally found to be disposed in lay- 

 ers of from one inch to several feet in thickness. These are said to be 

 siraiijied. The lowest, of which we have any knowledge, are non-stra- 

 tified ; that is, they are not divided by parallel planes, or they manifest 

 no tendency to split in one direction rather than in any other. The 

 stratified always preserve the same relative position in reference to each 

 other, so that they are never found to interchange places, except in a few 

 instances, where a local inversion, caused by some violent action from 

 below, has taken place. They also preserve the same mineral character 

 or the same fossils, by which they are at once identified all over the 

 world. It must however not be supposed that the strata are co-exten- 

 sive with the Earth, covering it every where alike, as an onion is cpver- 

 ed by its several coats. They vary very much in thickness and extent 

 in dilierent places, some being entirely wanting in some localities; but 

 still the invariable relative position of the rest to each other is preser- 

 ved. Their number, when all are taken together, is considerable, and 

 their collective tliickness is not less than several miles; these facts being 

 ascertained by following the course of some stream, which makes a nat- 

 ural section at right angles to the direction of their strike. Being con- 

 siderably inclined to each other, and making their appearance succes- 

 sively at the surface, they can be counted and measured, and the aggre- 

 gate can thus be obtained. In this way, following the course of the 

 Susquehanna from its mouth in the Chesapeake Bay along its numerous 



