224 THE MINERAL RESOURCES OF CECIL COUNTY 



the crystalline rocks and the Coastal Plain deposits, is a separation 

 in the character of the various road materials. As already described, 

 this line follows approximately the line of the railroads from the 

 Susquehanna to the Delaware line. To the northwest of the boun- 

 dary the road materials are crystalline rocks of various sorts described 

 in the following paragraphs, while to the south and east the material 

 suitable for the construction of good roads consists of gravels and the 

 oyster shells gained from the Bay. 



Among the various rocks distributed over the northern and western 

 portions of the county two are of particular value as road materials 

 and their areal distribution may easily be gained by a glance at the 

 geological map accompanying this report. These are the gabbro and 

 the granite. 



GABBRO. 



Beginning in the vicinity of Conowingo there extends eastward 

 a well-defined belt of trappean rock which is well adapted both by 

 its cementing and wearing qualities to the construction and main- 

 tenance of high-class macadam roads subject to considerable wear. 

 This material has been used somewhat in the repair of local highways 

 but in almost every instance it has not proved satisfactory for more 

 than a short time, either because the pieces used have been too large 

 or because the proper sized pieces have been put upon a road-bed 

 which had not previously been prepared properly to receive it. To 

 gain the best results from this gabbro rock it is necessary to use it in 

 layers a few inches thick distributed and carefully rolled one by one. 

 On the lower course the rock fragments should be between lj and 

 2-| inches in diameter, while the upper stone should be spread upon 

 the well-rolled lower courses, and the individual pieces for the upper 

 course should not be larger than from f to H inches in diameter. 

 When properly rolled the first course should be 4 inches thick and the 

 second, or upper, course 2 inches in thickness. When this manner 

 of distributing the road material is employed it is found that the 

 material from the gabbro belt of Cecil county will give most excel- 

 lent roads well adapted to the wants of the people. 



