PLACE AND MODE OF OCCURRENCE 463 



of flow ; the smaller ones may be flattened oval in shape. Even in the 

 weather-beaten face of the cliff the presence of white zeolitic minerals 

 filling the cavities is very noticeable. At a depth of 2 or 3 feet they are 

 usually quite fresh. 



Cross and Hillebrand divide the zeolites filling the amygdaloidal cav- 

 ities in the basalt of North and South Table mountains into two groups, 

 based upon their method of occurrence. The minerals of the first group 

 are laumontite and stilbite, and are to be found only on the floor of the 

 cavities. On the floor of many of the cavities, especially of the larger 

 ones, there is to be seen a very peculiar bedded deposit of yellowish or 

 reddish yellow color, which closely resembles a friable sandstone, but 

 which is shown to be composed of mixtures of these two minerals. 

 Included in the minerals of the first group are also to be mentioned 

 occasional minute, reddish spherules of thomsonite. These latter have 

 not been observed by the writer in the newly opened locality ; but with 

 this exception the minerals of the first group are quite in accordance 

 with the descriptions given by Cross and Hillebrand and may be passed 

 over without further comment. 



The second group is made to include all those that are not confined 

 to the floor of the cavities. They occur on the roof and sides, as well as 

 on the floor composed of zeolites of the first group, or they completely 

 line cavities that do not contain these bedded floors. The minerals of 

 this second group are thomsonite, stilbite, chabazite, analcite, apophyllite, 

 and mesolite, to which maybe added calciteand aragonite, as both these 

 minerals occur associated with the zeolites of the second group. 



Thomsonite 

 general description 



It is universally recognized that specimens of any given species com- 

 ing from the same locality usually have the same habit and general 

 appearance. This similarity affects crystal form and habit, color, luster, 

 size, and association, and is so marked that mineralogists do not hesitate 

 to identify localities by such peculiarities, such identification of localities 

 usually being entirely justified by the facts. Exceptions to this rule will 

 doubtless suggest themselves to the reader, but it is doubtful if a more 

 striking one can be found than is shown by the mineral thomsonite in 

 the particular locality under discussion. It would not be difficult to 

 select four or more specimens of this mineral coming from immediately 

 adjacent cavities so markedly different in habit and general appearance 

 as to suggest their occurring in widely different localities. In fact, one 



