158 H. Warth — Origin of Limestone. 



of embryonic crystals, the features of which so closely approach 

 those of chiastolite as to render the identity highly probable. This 

 • unconventional ' occurrence of chiastolite will be fully discussed in 

 the May ^Number of the Geological Magazine. 



EXPLANATION OF PLATE VI. 



Structural Types. 



Fig. 1. — A dioritic facies — but with pronounced hypabyssal features. 

 Hornblende relies, of somewhat raggy outline, but tending to idiomorphism ; 

 cleavages pencilled out in serpentine. Base made up of much altered 

 plagioclase as prisms and irregular plates, the clear interspaces being filled with 

 chloritic and serpentinous matter ; some calcite ; abundant apatite ; some 

 titaniferous iron-ore (with leucoxene) ; grains of pyrites ; and a little granular 

 epidote in plagioclase, of which it is a decomposition product. 



Fig. 2. — A picritic facies. The proportion of plagioclase is much lower than 

 for No. 1 ; it occurs not only in the base but as minute prisms enclosed 

 ophitically in hornblende. Clear patches represent crystals of original olivine 

 now altered to serpentine, which also envelops plates and granules of calcite. 

 Circular and elliptic ' lakes ' of serpentinous matter enclosed pcecilitic fashion 

 in the hornblende appear to represent original olivine. 



Fig. 3. — A coarse lamprophyric facies (camptonite). The idiomorphic 

 hornblende is, in the main, phenocrystal, though the presence of small felspar 

 prisms as enclosures in the hornblende is (for lamprophyres) an anomalous 

 feature. This ophitic relationship of the felspar to the hornblende recalls the 

 structure of typical dolerites (the hornblende playing the role of pyroxene in 

 dolerites) ; and it may be that the fluid intrusive, potentially doleritic, has 

 crystallized at a temperature too low to promote the formation of pyroxene, 

 i.e. at a temperature at which hornblende can form and remain stable ; in this 

 sense the camptonite may be an abortive dolerite. But the phenocrystal aspect 

 of the hornblende (with other hypabyssal features) insistently recalls the lampro- 

 phyres : the rock must be regarded as a variety of camptonite. 



FlG. 4. — A fine-grained hornblende lamprophyre (closely related to the 

 coarse camptonitic facies), in which the idiomorphic hornblende occurs as 

 minute laths. 



V. — The Origin of Limestone. 

 By H. Warth. 



.4 LTHOUGH the calcium oxide and magnesia of sedimentary 

 J\ limestone and dolomite must all be derived from pre-existing 

 igneous rocks, it is not so easy to trace the origin of the necessary 

 carbon dioxide. The solidified igneous rocks contain mere traces of 

 free carbon dioxide and of calcite. During their weathering carbon 

 dioxide is taken up from the atmosphere, replacing the silica of 

 calcium silicate and magnesium silicate minerals, and forming the 

 carbonates. Now, however, the quantity of carbon dioxide retained 

 in the atmosphere is exceedingly small. It is difficult to credit, for 

 instance that a layer of coal, 1-g- millimetres thick, covering the 

 surface of the globe, on being burnt would yield as much carbon 

 dioxide as the entire atmosphere now contains. A similar layer of 

 limestone containing an equivalent amount of carbon dioxide would 

 have a thickness of 5 millimetres. • This is out of all proportion 

 compared with the mighty beds of limestone and dolomite now 

 forming part of the earth's crust. Nor would the carbon dioxide 



