238 Pogue, Jr. — Geology and Structure of Volcanic Rocks. 



Thickness and age of the slate series. 



Thickness. — Nothing definite can be said about the thickness 

 of the volcano-sedimentary series. From the cross-section, 

 however, it appears probable that the series is from 2 to 4 

 miles thick. This estimate is given by no means as a final 

 figure, but with the hope that future work in the same province 

 may tarn it into something more definite, corroborative or 

 otherwise. 



Age. — Again, nothing final can be said about the age of the 

 slate series. It has generally been considered to be Pre-Cam- 

 brian. Volcanic rocks of a somewhat similar nature in the 

 South Mountain region of Pennsylvania occur beneath Cam- 

 brian sandstone.* As there is no evidence to the contrary, 

 the present series is provisionally correlated with the Pre- 

 Cambrian. 



Acknowledgments. 



In conclusion, the writer wishes to acknowledge his great 

 indebtedness to the advice and previous work of Dr. F. B. 

 Laney, whose report on the adjoining Gold Hill District, con- 

 taining many of the results herein set forth, is now in press. 

 Also the author desires to thank Professor Joseph Barrell for 

 suggestions in regard to the structure, and Professor L. V. 

 Pirsson for interest and advice during the entire investigation 

 and the preparation of this article. 



* Williams, G. H., The Volcanic Kocks of South Mountain in Pennsylvania 

 and Maryland, this Journal, v. xliv, 493-494, 1892. 



