28 
MUSEUM BULLETIN NO. 4, 
Alteration. These primary rocks are very slightly altered. 
Much of the feldspar is kaolinized, but not to any great extent, 
and a large amount of clear glassy sanidine and orthoclase is 
found. iEgirite-augite is usually quite unaltered, though some 
of it is changed to chlorite. The analcite is slightly changed in 
some specimens, and a replacement of this mineral by calcite 
occurs in varying degrees up to completion. Alteration of 
melanite has not been observed. Such changes as have taken 
place are those characteristic of the zones of weathering and 
oxidation, and no secondary minerals characteristic of the deeper 
zones are present. 
SECONDARY ROCK TYPES. 
(PYROCLASTICS). 
Preliminary Statement. 
The scope of the present paper does not admit of a detailed 
description of the many varieties of agglomerate and tuff studied 
in thin section. Certain types are peculiar, or are repeatedly 
found, and these merit a word of description. 
The lithologic terms used here in describing the pyroclastic 
rocks should perhaps be defined, as there seems to be some dis- 
agreement as to the exact meaning of the words tuff, agglomerate, 
and breccia. Tuff in this paper denotes a stratified deposit of 
the finer volcanic ejecta, of the general size of grain of sandstones. 
The coarser varieties are termed agglomerates. Breccia is used 
only to mean “broken rock,” as it is a term of too general applica- 
tion to be applied to a single class of deposits without some 
qualifying adjective. Even in the latter case it is not necessarily 
definitive, as a wide variety of pyroclastic rocks, of differing tex- 
tures and origins, may properly be spoken of as volcanic breccias. 
As in the case of sandstones and conglomerates, there is an 
intimate gradation between tuffs and agglomerates, and the 
varying proportions of fragments of different sizes precludes 
the possibility of rigid definition. In this paper, stratified pyro- 
clastic rocks having fifty per cent or more of their fragments 
great than six millimetres (one-fourth inch) are termed agglom- 
