MARYLAND GEOLOGICAL SURVEY 101 



plateau. This is traced mainly by means of a trail of boulders and its 

 limits are accordingly somewhat hypothetical. It probably enters 

 the State a short distance west of Sylmar and traverses the county 

 a little to the southeast of Rising Sun, Color a and Liberty Grove. Its 

 course is shown on the map in dark red. A rotten, yellowish red rock, 

 breaking in cuboidal blocks with aphanitic green centers, occurs at 

 the following points in the neighborhood of Rising Sun: three-six- 

 teenths of a mile north of the railroad on the highway which crosses 

 the Central Division of the Pennsylvania road, north of the town; 

 in the railroad cut northeast of the station and at other points to the 

 southwest. This was thought to be the weathered diabase and was 

 made use of in mapping the dike. 



South of this dike and roughly parallel with it occurs another but 

 coarser grained dike of the same material. It has already been men- 

 tioned as located north of Happy Valley Branch. At Williamson's 

 Point on the Susquehanna, one mile north of the State line, a 3 to 

 4 inches wide miniature diabase dike traverses the gneiss. 



These diabase dikes are the attenuated continuations of the great 

 intrusion of igneous material in the Triassic of the Atlantic coast. 

 Mt. Tom and Mt. Holyoke, the Palisades of the Hudson and the 

 Highlands of New Jersey represent the great masses of this intrusive 

 and extrusive body. In Pennsylvania this period of igneous activity 

 is represented only by numerous dikes and larger intrusive bodies 

 of diabase, some of which can be traced more or less continuously 

 across the state and into Maryland. The Cecil county diabase dikes, 

 therefore, although no external evidence of their age is furnished 

 within the county, must be of late Triassic age, because they are 

 part of a formation which to the northeast is intrusive in Triassic 

 sandstones and shales, 



PEGMATITE VEINS. 



Mention has already been made of the numerous pegmatite and 

 quartz veins which traverse all the formations indiscriminately. 

 Only those pegmatite or quartz veins which are exposed and exceed 

 fifty feet in width have been mapped. The abundance of the quartz 



