Notice of a Geological Model. 87 



impress on the mind and memory, is no mean part remaining to 

 be performed. Let it be remembered that facts are comparatively- 

 useless without arrangement ; that they are valueless if they are 

 not presented to the senses in an intelligible and accurate form. 

 The elements wherefrom to erect a geometric figure may be be- 

 fore us, but until we have truly constructed that figure from those 

 elements, our impressions as to its form and proportion are neces- 

 sarily vague and feeble. The materials wherewith to construct 

 a house or a ship may be all prepared with strict regard to their 

 individual dimensions, but as separate members they convey to 

 us no idea of the actual form of that house or that ship. It is 

 the art of the builder then to put together those materials ; and, 

 in like manner, the geologist, or the geological draftsman, or the 

 modeller, has to exercise his art, in putting together and exhibit- 

 ing in correct forms, the details he has labored so hard to collect. 

 Above all things, let him avoid distortions in drawing. It is in- 

 cumbent upon those who undertake to enlighten and instruct 

 others by diagrams, to exhibit those diagrams in true, and not in 

 false proportions. The master can no more hope to convey to his 

 pupil a right idea of a cube or a square, for instance, by repre- 

 senting in his diagram its height four or five times its breadth, 

 than can the draftsman in our science, expect to convey correct 

 notions of geological arrangement by a similarly defective process. 

 Returning to the model before us. The local elevations above 

 the level of tide, have been ascertained at a sufficient number of 

 points, particularly in the coal districts, to convey the prevailing 

 characters of the country. A number of these heights, are marked 

 on their proper sites upon the model. All of these were found by 

 spirit level and positive admeasurements. The Pennsylvania 

 canal on the east bank of the Susquehanna river, where it cuts 

 through the second mountain, is three hundred and twenty seven 

 feet above tide level in Chesapeake Bay. The Swatara river, 

 above Pinegrove, passes through the same mountain, thirty one 

 miles to the eastward, at the height of six hundred and nine feet 

 above tide. The prevailing elevation of the ridges which form 

 the north and south edges of the southern coal basin, is sixteen 

 hundred or sixteen hundred and fifty feet above tide water. As 

 a general remark, when casting a glance over the area here repre- 

 sented, we cannot but be struck with the comparative uni- 

 formity in their elevations, and the extensive maintenance of 



