134 GEOGNOSY AND GEOLOGY. 
operations. It not rarely occurs ; that cracks which start in lava currents 
become lined with crystallizations of specular iron, common salt; and 
sal-ammoniac. The sublimation of galena in stack furnaces, which is 
deposited in splendid crystals, is a fact of high importance. The infiltratior 
spoken of under this head must, perhaps, be distinguished from lateral 
secretion ; water, indeed, may in great part have leached the accompanying 
rocks, and have collected at the bottom, there, however, to be heated and 
driven up. Thus we do not have an immediate lateral secretion, but an 
ascending infiltration. This has not, indeed, been observed in veins; yet 
some plausibility is derived from the analogies furnished by the Carlsbad 
fountains and other springs, which deposit calcareous sinter or hydrated 
oxyde of iron, as also by the hot springs of Iceland, the Geyser, the 
Strocker, &c., which deposit silex and chalcedony. These deposits take 
place not only on the exposed surface, but also on the inner walls of the 
fountains. 
Although some of these theories may be capable of explaining particular 
phenomena, yet we may not assume that the causes they suggest are the 
only ones; it is exceedingly probable, that of many agencies, both past and 
present, which have played and are still playing their part in the bosom of 
the earth, we are entirely ignorant. 
17. Fossil Organic Remains, their Relation to the Existing Organic 
World, and their Significance in a Geological Point of View. 
Petrifactions are organisms more or less perfectly preserved, and partly 
or entirely converted into stone. It must not be understood, however, that 
an organic substance has really been transformed into an inorganic, but 
only that as particles of the former have been removed by decomposition 
or other causes, their places have become filled by mineral matter. Some, 
however, must be considered as simple transformations of organic matter, 
as is shown in the conversion of wood into coal. 
Petrifactions in general may be considered : 
1. In a natural history point of view, by which they are classified and 
described. Their most natural classification is that which interpolates the 
different species of animals and plants into the present zoological and 
botanical series. As many of them are forms entirely foreign to those 
which now exist on the surface of the earth, such an arrangement is 
capable of furnishing the most desirable and interesting conclusions with 
reference to the development of organic life. 
To every one who has been at all occupied with a special study of 
botany or zoology, the fact will be familiar, that among closely related 
classes, orders, families, and genera, there occur species which it is 
difficult to refer to one division rather than to another; species which 
appear to form the actual transition from one such division to the rest. In 
other cases the reverse is seen, and groups stand out isolated from ail others; 
species even occur which are apparently disconnected with their fellows, 
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