EXCURSION XI. 157 



Allport of Birmingham (Geol. Mat/., Jan., 1872). Having 

 submitted thin slices to high magnifying powers, a singular 

 internal arrangement was brought to light. In the accom- 

 panying woodcut (fig. 29) we reproduce one of Zirkel's 

 figures. Mr. Allport's are still more elaborate and beauti- 

 ful. Describing the section or slice of a specimen from 

 the Corriegills shore, Mr. Allport says, "It is seen to 

 consist of an amorphous glassy base, containing many long 

 slender prisms of a green pyroxenic mineral, which are 

 occasionally isolated, but generally form the axes, to which 

 are attached innumerable minute pale green crystals, arranged 

 in exquisitely beautiful groups. Some are wonderfully like 

 fronds of ferns; others have the closest resemblance, both in 

 form and colour, to microscopic fresh- water algae; in fact, 

 the field of view seems to be crowded with minute 

 ferns. . . . The glassy base has a pale yellowish tint 

 in the open spaces between the groups; but under a higher 

 power the colouring matter is resolved into a mass of trans- 

 lucent granules and minute crystals, the latter being much 

 smaller than the acicular prisms which form the fern-like 

 groups. A comparison of many specimens, affording grada- 

 tions in size, shews that all these crystalline particles consist 

 of the same pyroxenic mineral as the larger prisms which 

 form the axes. . . . The aggregations of acicular prisms 

 are invariably surrounded by a border of clear colourless 

 glass. . . . This glass has not the property of double 

 refraction; but this power is possessed by the pyroxenic 

 acicular prisms, which hence exhibit colours. These prisms 

 are about one-thousandth of an inch long and one-ten- 

 thousandth broad." 



Similar dendritic forms, and others of a bizarre character, 

 were observed in sections of all the pitchstones examined, 

 and even in felspar crystals and imperfect crystals of quartz. 

 They have a striking resemblance to the dendritic forms in 

 the crystals of snow and ice, which are, in fact, formed 

 under like conditions of solidification from a state of free 



