chap. i. Calcareous Matter entangled in Lava. 1 5 



the purest specimens analysed, with a view to discover, 

 considering their volcanic origin, whether they con- 

 tained much magnesia ; but only a small portion was 

 found, such as is present in most limestones. 



Fragments of the scoriae embedded in the calcareous 

 mass, when broken, exhibit many of their cells lined 

 and partly filled with a white, delicate, excessively 

 fragile, moss-like, or rather conferva-like, reticulation 

 of carbonate of lime. These fibres, examined under a 

 lens of one-tenth of an inch focal distance, appear 

 cylindrical ; they are rather above the 10 1 00 of an inch 

 in diameter ; they are either simply branched, or more 

 commonly united into an irregular mass of net-work, 

 with the meshes of very unequal sizes and of unequal 

 numbers of sides. Some of the fibres are thickly 

 covered with extremely minute spicula, occasionally 

 aggregated into little tufts ; and hence they have a 

 hairy appearance. These spicula are of the same dia- 

 meter throughout their length; they are easily detached, 

 so that the object-glass of the microscope soon becomes 

 scattered over with them. Within the cells of many 

 fragments of the scoriae, the lime exhibits this fibrous 

 structure, but generally in a less perfect degree. These 

 cells do not appear to be connected with one another. 

 There can be no doubt, as will presently be shown, that 

 the lime was erupted, mingled with the lava in its fluid 

 state ; and therefore I have thought it worth while to 

 describe minutely this curious fibrous structure, of which 

 I know nothing analogous. From the earthy condition 

 of the fibres, this structure does not appear to be related 

 to crystallisation. 



Other fragments of the scoriaceous rock from this 

 hill, when broken, are often seen marked with short and 

 irregular white streaks, which are owing to a row of 

 separate cells being partly, or quite, filled with white 



