IN ROCKS OF DISSIMILAR ORIGIN. oon 
Carbonate of lime may occur in either eruptive or sedimentary 
rocks—in the former as cements or other infiltrations ; in the latter 
case either constituting entire rock masses or forming cements, &c. 
From this tabulation we see that felspars and quartz are the 
principal minerals common both to pyroclastic and clastic rocks, 
while it is quite possible for any of the other minerals, or alteration- 
products from them, to be also common to the rocks of both classes. 
(6) The nature of the changes which the foregoing minerals suffer 
from the action of heat are somewhat variable. 
The felspars may become fused superficially, or reduced to a com- 
plete state of fusion throughout. Microscopic examination of per- 
tectly fused felspar shows it to be a thoroughly amorphous substance, 
possessing the optical character of homogeneous glass. 
Lacunz of glass are common in many of the minerals constituting 
volcanic ejectamenta, and it is not uncommon to find fused surfaces 
on these crystals. Moreover the minerals in many lavas also contain 
glass lacunz, so that frequently no distinction can be made between 
the minerals occurring in lavas and those met with in volcanic ejec- 
tamenta. This is not to be wondered at, since ashes, lapilli, &c. are 
derived at times from the disintegration of lava, or possibly more 
often from the bursting-up by volcanic explosions of viscid magmas 
similar to those from which the lavas are derived. 
(7) An examination of recent lavas, erupted both beneath the sea 
and on land-surfaces, shows that no appreciable difference exists 
between them. 
(8) Both lavas, intrusive sheets of molten rock, and volcanic 
ejectamenta may occur interbedded with ordinary sedimentar 
rocks ; and except from alteration of the adjacent sediments at the 
contact of the lavas at their lower surfaces, and at the upper and 
lower surfaces of the intrusive sheets, there is no evidence to be pro- 
cured in the field to show whether a certain bed represents a volcanic 
ash, a tuff, or an ordinary detrital sediment such as a felspathic grit. 
The intercalation of clastic rocks with lava-flows lends considerable 
probability to the assumption that such rocks may be of pyroclastic 
origin; but it does not afford conclusive evidence, since we occa- 
sionally meet with ordinary sedimentary deposits occupying similar 
positions. 
(9) In view of the foregoing considerations, I think we ma 
assume that :—Neither the angularity of fragments and crystals, the 
rounding of fragments and crystals, the mineralogical or lithological 
character of fragments and crystals (except perhaps the fusion of 
surfaces of crystals, their envelopment in glass, or the presence of 
separately fused fragments of glass), the inclusions in crystals, the 
presence or absence of cementing matter, the petrological associations, 
nor the mode of occurrence afford any certain clue to the origin of 
fragmentary rocks, and that assertions hitherto made regarding 
their pyroclastic origin are incapable of demonstration, except in 
instances where it is known that no submergence of land has taken 
place in particular areas since certain periods of volcanic activity, 
or in instances in historical times in which showers of ashes have 
