Part ITI. § iv.] MICROSCOPIC CHARACTERS OF ROCKS. 107 
Moreover, in clastic calcareous rocks, owing to their liability to 
alteration by infiltrating water, there is.a.tendency to acquire an in- 
- ternal crystalline texture. At the time of formation little empty 
spaces lie between the component granules and fragments, and 
according to Mr. Sorby, these interspaces may amount to about a 
quarter of the whole mass of the rock. They have very commonly 
been filled up’by calcite introduced in solution. This infiltrated 
ealcite acquires a crystalline structure like that of ordinary mineral 
veins. But the original component organic granules also themselves 
become crystalline, and, save in so far as their external contour may 
reveal their original organic source, they cannot be distinguished from 
mere mineral grains. In this way a cycle of geological change is 
completed. The calcium carbonate orginally dissolved out of rocks 
by infiltrating water and carried into the sea is secreted from the 
oceanic waters by corals, foraminifera, echinoderms, molluscs and 
other invertebrates. ‘The remains of these creatures collected on the 
sea-bottom slowly accumulate into beds of detritus, which in after 
times are upheaved into land. Water once more percolating through 
the caleareous-mass gradually imparts to it a crystalline structure, 
and eventually all trace of organic forms may be effaced. But 
at the same time the rock once exposed to meteoric influences 
is attacked by carbonated water, its molecules are carried in solution 
into the sea, where once again they will be built up into the frame- 
work of marine organisms. : 
K. ALTERATION OF Rocxs.—One of the most important revelations 
of the microscope is the extent to which rocks have undergone 
alteration through the influence of infiltrating water. The nature 
of some of these changes is described in subsequent pages. It may 
be sufficient to note here a few of the more obvious proofs of altera- 
tion. Threads and kernels of calcite running through an eruptive 
rock, such as granite, dolerite, or trachyte, are a good index of in- 
ternal decomposition. They usually point to the decay of some lime- 
bearing mineral in the rock. Some other minerals are likewise 
frequent signs of alteration, such as serpentine (often resulting from 
the alteration of olivine, see Fig. 6), chlorite, epidote, limonite. In 
many cases, however, the decomposition products are so indefinite in 
form and so minute in quantity, as not to permit of their being satis- 
factorily referred to any known species of mineral. For these indeter- 
minate but frequently abundant substances, the following convenient 
short names have been proposed by Vogelsang to save periphrasis, 
until the true nature of the substance is ascertained. Veredite—green 
transparent or translucent patches, often in scaly or fibrous aggre- 
gations, of common occurrence in more or less decomposed rocks 
containing hornblende, augite, or olivine: probably in many cases 
serpentine, in others chlorite or delessite. Ferrzte—yellowish, red- 
dish, or brownish amorphous substances, probably consisting of 
peroxide of iron either hydrous or anhydrous, but not certainly 
referable to any mineral, though sometimes pseudomorphous after 
