'68 



DYNAMICAL GEOLOGY. 



is sustained by DaubreVs remark respecting the claj r -porphyry of Germany containing 

 quartz crystals and kaolinized feldspar crystals, that. " the change must have been pro- 

 duced before the rock was completely cooled." 



In these cases, the igneous rock at first was so far' consolidated that its minerals 

 took their crystalline condition, and afterward came the change. It follows, thence, 

 that the moisture could not make these hydrous silicates out of the material present, 

 while the temperature of fusion existed, but only after cooling was partly completed. 

 It is so in the lavas of a volcano; although moisture is so abundant as to furnish vapors 

 freely from the vent, it causes no changes on passing through the lavas. In fact, 

 these hydrous minerals lose their water, or most of it, below the temperature of igni- 

 tion, and it is natural that they should require, notwithstanding the pressure, a com- 

 paratively low temperature. Confinement with the minerals for a long period during 

 the slow cooling seems to be the condition under which the metamorphism and the 

 attendant making of zeolites, agates, etc., takes place. 



In the making of chlorite schist by metamorphism, the moisture was present in the 

 sediments, and was retained in consequence of the pressure, and the low degree of tem- 

 perature under which metamorphism took place, so that the case is different from that 

 of chloritic doleryte. 



As to the euphotide, the conditions, even if similar, were in some important respects 

 different. The rock, as originally made, had the chemical constitution of true doleryte, 

 and if pyroxene was present in it mainly, instead of hornblende, it had its mineral 

 constitution. But the labradorite in doleryte has not been found changed to saussurite, 

 while it is so over great areas in euphotide. 



Dolomization. — The making of dolomite out of limestone materials was, according 

 to Von Buch's theory, a process of metamorphism ; for it attributed the change to the 

 action on existing limestones of magnesian vapors attending the eruption of augitic 

 igneous rocks. But such an effect, if taking place, would necessarily be local; and 

 magnesian vapors are not thus generated. Magnesian limestones are, in fact, some of 

 the most extended of rock formations, and constitute a considerable part of the uncrys- 

 talline limestones of the world. Moreover, they bear evidence that they have become 

 magnesian, for the most part, in the process of their formation, and not through subse- 

 quent change, and the crystalline variety (as that of the Green Mountain region from 

 Vermont to New York) is only a metamorphic rock of this nature. That this is so, 

 may be inferred (1) from the extent of such limestones; (2) from their usual compact- 

 ness, which would not be so common if they were a result of chemical alteration (since 

 magnesium has much greater density and less atomic weight than calcium, and, there- 

 fore, a limestone is diminished much in bulK. by the change to dolomite — the diminu- 

 tion being about one eighth); and (3) from the fact, announced in the author's Explor- 

 ing Expedition Geological Report (18-49), that a white, compact coral limestone from the 

 reef-rock of the elevated coral island. Metia, north of Tahiti, has nearly the composi- 

 tion of true dolomite, as determined in an analysis by B. Silliman (the analysis obtain- 

 ing, from 100 parts, 61-93 of calcium carbonate and 38 07 magnesium carbonate). Fur- 

 ther, the texture of the ordinary magnesian limestones usually indicates that they are 

 not simply of chemical origin, for it is quite unlike that of travertine and any purely 

 chemical calcareous deposit. 



The general character of the Metia reef-rock, its fineness of grain and compactness, 

 and its position, led to the inference that it had been formed from the coral mud that 

 had been deposited in the shallowing lagoon of the small coral island. The occurrence 

 of such mud is common under these circumstances, and not unfrequently beds of gyp- 

 sum are formed in it, as stated on page 235. Since reef-forming corals contain very 

 little magnesia (page 60), the facts seemed to sustain the inference (as stated in the 

 report referred to) that the coral mud became magnesian though the magnesian salts 

 of the ocean (the chloride chiefly, or both the chloride and sulphate), at the time of its 

 solidification, and without the aid of heat — slow action through centuries accomplishing 

 what is not possible by quicker methods. In such a shallowing lagoon, holding con- 

 centrated sea-water, the conditions night be favorable for the action, although without 



