106 Transactions of the Royal Microscopical Society. 



one of several specimens isolated from others, and in which an 

 organic structure will scarcely be denied. It will be seen that a 

 number of branches spring from a slender upright stem, usually 

 exactly opposite, but occasionally alternate. At the point from 

 whence these branches spring there is a slight thickening of the 

 stem, scarcely amounting to a knot, and no positive structure of 

 any kind can be discerned either in branches or stem. In examin- 

 ing the slide, whole branches and fragments were frequently seen 

 separate from any stems, and in some of the compound masses the 

 stems were often wanting in places, or too indistinct for certain 

 vision. Fig. 1, Plate XXVII., is magnified about 300. 



Another formation, as shown in Fig. 2, has a decidedly organic 

 aspect. It is a stem with short hair-like branches in more or less 

 confusion, x 90. Fig. 3 is an exceedingly delicate formation, like 

 a slender feathered shaft, x 160. 



Fig. 4 represents a double spiral, but this could not be seen 

 sufficiently well to be quite certain of the real shape, x 160. 



Fig. 5 represents one of many threads of various lengths with a 

 spiral twist, x 160. 



Fig. 1 must not be taken as representing a complete object ; it 

 is only a portion of a much longer one, and the shde gave no certain 

 indications of how great the full length of this and other objects 

 might be. In point of size Fig. 1 occupies a mean place, some 

 similar objects being twice as big, and others much less. 



To the question. What are these things ? the writer is not in a 

 position to give any determinate answer. Their aspect is that of 

 minute vegetable fossils, possibly Algie, but this is merely conjec- 

 tural. The object of submitting this paper to the Society is to 

 draw the attention of others who may have opportunities of 

 observing slices of fire opal from Mexico or elsewhere, to the 

 probability of their discovering objects similar or divei-se, but which 

 may throw further light on the subject. It has been thought 

 better to give no name to the objects until their nature is under- 

 stood. 



Opals are hydrates of silica, and there is nothing in their com- 

 position or probable mode of formation that renders their containing 

 fossils improbable. Dana states : " Opal consists of soluble silica 

 and 5 to 12 per cent, of water." 



Several specimens of Fig. 1 are so placed in the opal slice as to 

 admit of very distinct vision. Fig. 2 represents the plainest piece 

 of that form. Some like it of considerable length were bigger, 

 and others less. 



No objects of the kind could be found in a slice of precious 

 opal in the matrix from another part of Mexico. 



