546 GEORGE DAVIS LOUDERBACK 



considerable time for their accomplishment. To bring this more 

 distinctly before the mind, we may summarize the more important 

 events connected with this interval. 



i. There were at least three distinct periods of igneous activity, 

 in each of which rocks of more or less variety and in considerable 

 abundance were produced. These three periods have not been 

 found to overlap, but everywhere have a definite order of eruption 

 over large areas, and probably represent distinct intervals of time. 



If these three groups are in any way genetically connected, they 

 must represent deep-seated and general differentiation, and we may 

 note that the basic (to intermediate) group appeared first and in 

 greatest abundance, followed later by the ultrabasic and the com- 

 paratively acid groups, apparently in the order named. Within each 

 group there has been distinct and often considerable differentiation, 

 the products being related in lithological characters and by grada- 

 tional forms. Compared with the hypothetical differentiation between 

 the groups, this intra-group differentiation is more local, less com- 

 plete, more evident, and does not make the same time demand upon 

 the period under discussion. 



The basic series may have been in part contemporaneous with the 

 deposition of the members of the Dillard series. 



2. The degree of lithification of the Dillard, which is one of its 

 most marked characteristics, is considerably higher than that of the 

 Myrtle. The Dillard type of alteration, the cementation and quartz- 

 veining, could only have been produced at some depth, and the 

 abrupt change on passing to the Myrtle, and the absence of com- 

 parable alteration in the lowest Myrtle, show not only that the altera- 

 tion was pre-Myrtle, but that the upper unaltered layers of the Dillard 

 had been removed before the deposition of the basal beds of the 

 Myrtle group. 



3. The general strike and the elongation of the Dillard area are 

 the same as those of the Myrtle Creek area. This is apparently due 

 to the orogenic disturbances in post-Myrtle time (including pre- 

 Eocene and post-Eocene movements), which affected both groups in 

 the same way. But the marked irregularities of attitude in the 

 Dillard near the base of the Myrtle, and its divergence from the 

 latter within short distances at places where, within the Myrtle, the 



