MARYLAND GEOLOGICAL SURVEY 173 



This is the only formation of the Columbia group which has yield- 

 ed fossils of any kind in Cecil county. There are developed in the 

 Talbot formation lenses of drab-colored clay, carrying stumps, roots 

 and knees of cypress, together with leaves and other vegetable re- 

 mains. 



Interpretation of the Geological Record, 

 sedimentary record of the crystalline rocks. 



The crystalline rocks of Cecil county have suffered so many dis- 

 turbances since their formation that they now almost defy interpre- 

 tation, and their history is consequently imperfect and fragmentary. 

 The oldest rock in the series, and therefore the most ancient in Cecil 

 county appears to be the mica-gneiss, which is either pre-Cambrian 

 or Cambrian-Silurian in age. This mica-gneiss is the product of meta- 

 morphism acting upon finely conglomeratic, arkosic, sandy and argil- 

 laceous sediments. 



These materials were probably accumulated just off-shore during 

 the denudation and subsidence of a pre-Cambrian continent. Subse- 

 quently this sedimentary series suffered elevation with severe fold- 

 ing, intrusion by igneous masses and metamorphism. Into this sedi- 

 mentary formation eruptive masses have been intruded during periods 

 when these were thicker than at present or more heavily covered with 

 sediments which were later removed by erosion. These igneous rocks 

 were apparently injected during three distinct periods of disturbances, 

 and were introduced in the order — granite ; rhyolite ; gabbro, pyroxen- 

 ites and peridotites. These eruptions were accompanied by differ- 

 entiation and solidification of the magmas. Then or subsequently, 

 the rocks suffered metamorphism, which has produced a laminated or 

 gneissic structure in the granite and gabbro and converted the pyrox- 

 enites and peridotites into amphibole schists, serpentines and steatite 

 schists. 



This early period of sedimentation and intrusion of igneous intru- 

 sives was followed by long ages during which the land probably stood 

 above sea-level, a victim to the long continued action of subaerial denu- 

 dation. If at any time during the long interval between the deposi- 



