Pirsson — Microscopical Characters of Volcanic Tuffs. 193 



being more and more studied by modern petrographic methods, 

 and as previously noted, there are all gradational varieties of 

 transition from them into tuffs. 



Classification of Material. 



Fragtnental volcanic material may be roughly divided for 

 convenience into pieces the size of an apple and upward as 

 volcanic bombs; those the size of nuts, lapilli ; ones like 

 small peas or shot, volcanic ashes ; while the finest is volcanic 

 dust. The divisional points may be supposed to lie midway 

 between these sizes. The coarser material, the bombs, lapilli, 

 and much of the ash, may fall around and near the vent and 

 produce beds of breccia ; the lighter ashes and dust, supported 

 and carried by air currents, tend to fall after these and at 

 greater and greater distances from the vent ; their compacted 

 material is known as tuff. Naturally all gradations both in 

 vertical and horizontal directions will occur between tuffs and 

 breccias. The term volcanic conglomerate, sometimes used in 

 the place of breccia, should be restricted to water-laid con- 

 glomerates consisting of volcanic materials which exhibit 

 erosional wear. In the use of volcanic agglomerate it would 

 be best to follow Geikie* and confine the use of the term to the 

 tumultuous assemblage of blocks, often of large size, which 

 may be found filling the upper portion of old volcanic con- 

 duits. 



It is obvious from the sizes mentioned that the determina- 

 tion of volcanic breccia is a matter of megascopic study, best, 

 perhaps, performed in the field. If the microscope is to 

 throw any light on rocks which are supposed to have this ori- 

 gin it will be chiefly by a study of the tuffaceous filling 

 between the larger pieces, aided perhaps by the nature of their 

 contours. It is, therefore, the tuffs which must be the subject 

 of microscopical study, and what knowledge they afford can 

 be easily carried over to include the breccias. 



Composition of Tuffs. — Tuffs are composed of three things: 

 «, glass; b, crystals of individual minerals; and c, fragments 

 of rocks which may be holocrystalline or partly glassy. 

 While all degrees of purity and admixture of these three occur, 

 accordingly as one or the other predominates or gives decisive 

 character to the material, we may distinguish vitric tuffs, 

 crystal tuffs, and lithic tuffs; the use pf "vitric" and "lithic" 

 is suggested instead of the more common "glassy" and "stony" 

 in order to avoid the misapprehension that the outward appear- 

 ance of the material is referred to. Tuffs also may be fresh, 

 altered, or metamorphosed, and each of these three stages 



* Text-book of Geology, 4th Ed., 1903, p. 173. 



Am. Jour. Sci. — Fourth Series, Vol. XL, No. 236. —August, 1915. 

 13 



