490 Johnston, Merwbi, and Williamson — 



section, and would have roughly the density observed by 

 Lacroix (2 - 58-2"70), but the maximum double refraction of 

 such sections would be O100 whereas Lacroix observed only 

 0'020. We may conclude, then, that until the precise nature 

 of these pisolites is established the term " ktypeite " can have 

 no definite significance.* 



3. Conchite. — This alleged form, occurring in certain shells, 

 has properties too close to those of aragonite for it to be defi- 

 nitely distinguishable^ and so needs no further consideration. 



4. Lublinite. — There have been a number of papers on 

 another alleged form, called lublinite ; it occurs only as felt- 

 like intergrowths of very small acicular crystals (not over 

 - 02 mm in breadth) with very oblique extinction. Quite recently, 

 however, a critical study of the observed characteristics of this 

 substance has been published by Quercigh,^: who also reviews 

 the previous papers dealing with lublinite. He has been able 

 to observe in lublinite a perfect cleavage the direction of which 

 is so related to the length of the needles that he considers the 

 needles to be rhombohedral crystals greatly elongated parallel 

 to a set of edges. § The observed density and refractive indices 

 are those of calcite within the limits of error — large in some 

 cases — of the measurements. The crystals when heated to 

 450° do not recrystallize ; when boiled with cobalt nitrate they 

 give the reaction for calcite. Quercigh has, therefore, in our 

 opinion, conclusively established that " lublinite " is identical 

 with calcite. 



5. " A?norphous" CaC0 3 . — One still encounters statements 

 which imply that "amorphous" calcium carbonate is a distinct 

 form in the same sense that the several crystalline forms are 

 distinct ; but all recent evidence bearing on this general ques- 

 tion favors the view that the special properties of such u amor- 

 phous " material are dependent upon the extreme fineness of 

 grain of the separate particles (as precipitated initially) and 

 upon the mode in which these have come together to form the 

 aggregates large enough to be examined under the microscope. 

 Moreover it is, in the present instance, a subsidiary question 

 whether these ultimate grains are really crystalline or are of 

 the nature of supercooled liquids ; even if crystalline, they are 

 so very small, and hence the curvatnre of their surface is so 

 great, that the forces acting at the surface may be large enough to 

 mask the effects of the vectorial crystal forces within the par- 

 ticle. Again the extent of surface in proportion to their mass 



* See also Vater's discussion, Z. Kryst., xxxv, 149, 1902. 



fR. Brauns, Centr. Min. Geol., 1901, 134 ; H. Vater, Z. Kryst., xxxv, 149, 

 1902. 



JE. Quercigh, Rivista Min. Crist, ital., xliv., 65-94, 1916; who gives 

 references to all the previous papers. 



§ The possibility of an organic origin should not be overlooked. 



