204 Pirsson — Microscopical Characters of Volcanic Tuffs. 



clearly fall into one or the other of these three classes. While 

 many will, the majority of these rocks will be found to be in- 

 termediate in character ; for all gradations between the three 

 will be found in nature, with the exception that tuffs composed 

 of glass dust with stony ash particles, but devoid of individual 

 mineral crystals, must be extremely rare if indeed they can 

 occur. The most common kinds are those containing in 

 variable proportions all three ingredients ; such transitional 

 character should be taken into consideration in determining 

 and naming them. 



Tuffaceous Sedimentaries. 



"Where tuffs have fallen into water, or have been quickly 

 washed down into it after their deposition, they may not have 

 been essentially altered in composition or characteristic micro- 

 textures, and although, perhaj)S, beautifully stratified their 

 recognition by microscopic study may be relatively easy. In 

 such cases also they may grade into, or be alternated with, dis- 

 tinct volcanic conglomerates or breccias, and the microscopic 

 studies of the latter in the field may greatly aid in their deter- 

 mination, especially when the tuffs are altered. But when 

 tuffs have been exposed to weathering for some time, so that a 

 considerable amount of the alterations, which they so readily 

 undergo and which will be described later, has occurred in 

 them and they are then washed down, mingled with greater or 

 lesser quantities of ordinary land waste, and deposited anew, it 

 is often very difficult to decide in such beds whether tuffaceous 

 material is present or not. Under such circumstances the glass 

 shards are usually destroyed and with them the characteristic 

 vitroclastic structure. If, fortunately, they are found, then 

 decisively tuffaceous material is present. If not, search should 

 be made for ash-sized particles of pumice, which may exhibit 

 the vesicular structure and may possibly still prove to be of 

 glass ; for crystals with the peculiarities mentioned under 

 crystal tuffs ; and for tiny fragments like those of lithic tuffs. 

 If no glass can be found, or proved to have been present, the 

 determination cannot be certain if it rests on only one observed 

 characteristic; it can only become so when a sufficient assem- 

 blage of them is shown to be present. As petrographic studies 

 are progressively made of the later bedded rocks of the region 

 of the American Cordillera, it is certain that vast amounts of 

 tuffaceous sediments will be found in them ; in studying them 

 criteria of the nature advanced in this paper will be found 

 most useful. 



Alteration of Tuffs. 



It is the exception, rather than the rule, to find tuffs in the 

 condition in which they are when freshly deposited. For one 



