MARYLAND GEOLOGICAL SURVEY 41 



were not published until the fall of 1901. Dr. Leonard agrees with 

 Dr. Grimsley in the view that the acid and basic rocks have broken 

 through more ancient gneisses but differs from him in concluding 

 that the basic and ultra-basic varieties have been cut by extensive 

 dikes of granite and probably by the granite mass itself. It is con- 

 sidered by him that there is a gradual passage of the granite into the 

 diorite and that no line of separation can be drawn between them. 

 His study of the geological relations and occurrences also seemed to 

 show that the rocks of the area were not all formed at the same time 

 but that the region was the scene of eruptive activity for a consider- 

 able period during which the different types were produced. The 

 norites and gabbros are regarded as the first to be erupted and 

 the diorites as probably formed at the same or nearly the same time. 

 The age of the granites is somewhat uncertain. It seems probable 

 however, that they are younger than the norites and the diorites 

 though the difference in age cannot be great. The most basic rocks, 

 the peridotites and pyroxenites, were erupted at a later period than the 

 norites and gabbros through which they have broken. Some of the 

 pyroxenites, however, apparently form peripheral f acies of the norite 

 and probably belong to the same age as the latter. The pegmatites 

 and acid dikes are younger than most of the peridotites and pyroxen- 

 ites. Thus the whole complex of igneous rocks is thought by Dr. 

 Leonard to consist of facies of a, single magma uniting to form a 

 geological unit, though it is not considered that this view requires 

 the supposition that all of the rocks Avere formed either at the same 

 time or by a continuous eruption. 



Prior to the completion of the detailed work on the basic rocks by 

 Dr. Leonard the crystalline rocks of the entire county were mapped 

 provisionally by Dr. Edward B. Mathews in the summer of 1896 and 

 the results of this reconnaissance were shown in the geological map of 

 Maryland published in Volume I of the reports of the Maryland Geo- 

 logical Survey. 



The Potomac Group. 



The earliest paper of first rate importance on this subject was pub- 

 lished by Professor George H. Cook in 1868. Cook at that time was 



