192 Pirsson — Microscopical Characters of Volcanic Tuffs. 



porphyries" by Mugge,* with admirable plates, but these are 

 not always accessible to the student, neither are they written 

 in a form convenient for his use. 



The writer during tbe past twenty odd years has had occasion 

 to study tuffs in a number of cases in his own researches on 

 particular areas,f and in supervising the work of students in 

 his laboratory who were engaged in research on material col- 

 lected in the preparation of their theses for the doctor's degree, 

 he has had opportunity for the study of many more.:}: Some- 

 times the material under investigation was fresh distinct vol- 

 canic ash or tuff, but it has often been a question to be decided, 

 if possible, whether certain older, more or less altered, inter- 

 bedded rocks were volcanic tuffs or not. In the latter case it 

 has then been necessary to start from the unaltered typical 

 varieties and trace the various gradations into those which 

 have been changed. During the course of this experience 

 much has been learned, the details of which are not given in 

 the various publications cited ; only the final decisions arrived 

 at. It has been thought, therefore, that a succinct account of 

 some of these observations combined with the results of other 

 penologists would be of interest and value to students of 

 petrology ; more especially since the sedimentary rocks are 



* Untersuchungen ueber die " Lenneporphyre " in Wcstfalen und den 

 angrenzenden Gebieten," O. Miigge, Neues. Jahrb. fur Min., B. B. viii, p. 

 533—721 1893 



\ Castle Mountain Mining District, Bull. 139, U S. G. S. 1896, pp. 73, 78, 

 127 ; Judith Mts., Mont., 18th Ann. Eep., U. S. G. S., Pt. Ill, 1898, p. 482 ; 

 Igneous Rocks of the High wood Mts., Bull. 237, 1905, pp. 56, 158. 



% H. E. Gregory : Contributions to Geology of Maine, Pt. II, Bull. 165, 

 U. S. Geol. Surv., 1911, pp. 120-131. 



Jos. Barrell : Geology and Ore-deposits of the Elkhorn Mining Dist., 

 Mont., 22d Ann. Rep. U. S. Geol. Surv., Part II, 1902, p. 523. 



F. D. Laney: Gold Hill Mining Dist., Bull. 21, North Carolina Geol. 

 Surv.. 1910. pp. 29-37. 



J. E. Pogne, Jr.: Cid Mining Dist., Bull. 22, North Carolina Geol. Surv., 

 1910, p. 43 et seq, 



D. D. Cairnes : Wheaton Dist., Yukon Terr., Mem. 31, Geol. Surv. Canada, 

 1912, pp. 61-69. 



G. P. Loughlin : Gabbros and Associated Rocks at Preston, Conn., Bull. 

 492, U. S. Geol. Surv., 1912, p. 67. 



H. H. Robinson: San Franciscan Volcanic Field, Ariz., Prof. Paper 76, 

 U. S. Geol. Surv., 1913. 



R. D. Crawford: Geol. and Ore-deposits of Monarch and Tomichi Dists. , 

 Col., Colo. Geol. Surv., Bull. 4, 1913, p. 175. 



M. E. Wilson : Kewagama Lake Map- Area, Quebec, Geol. Surv. Canada, 

 Mem. 39, 1913, p. 45. 



M. Y. "Williams : Arisaig'-Antigonish Dist., Nova Scotia, Geol. Surv. 

 Canada, Mem. 60, 1914, p. 118. 



C. W. Drysdale : Geology of the Franklin Mining Camp, Brit. Col., Mem. 

 56, Geol. Surv. Canada, 1915, pp. 96 and 128. 



A. M. Bateman : Geology and Ore Deposits of the Bridge River District, 

 British Columbia, Memoir. Geol. Surv. Canada, in press. 



Bruce Rose : Geology of the Savona District, British Columbia, Memoir, 

 Geol. Surv. Canada, in press. 



