70 ROCK-METAMORPHISM. 



vage-solid : compare Allport's figure with our fig. 2. We there- 

 fore cannot accept this case, any more than the others, as an 

 example of peridote, unless in the condition of a pseudomorph. 

 We have been obligingly favoured by Mr. JohnjYoung, of 

 the Hunterian Museum of Glasgow, with a numberV speci- 

 mens of trap rocks from the Clyde district the ^same rocks 

 which supplied Mr. Allport with several of his specimens. From 

 the specimens which Mr. Young sent us we have had prepared 

 several microscopic sections ; but in none do we observe any 

 thing opposing the above conclusion. Many of the crystals 

 contained in them (showing peridotic colours) exhibit irregular 

 rhomboidal sections, which, we consider, prove that the longi- 

 tudinal or vertical axis of the sections is inclined ; moreover 

 several of them exhibit prismatic rhombic cleavage. The mineral 

 we must therefore consider to have been originally augite or 

 hornblende *. 



In our present investigations we have been materially assisted 

 by Professor Hull, who, though aware that we were disposed 

 to take a view different from his, has kindly allowed us to exa- 

 mine the sections represented in the " Report " with which his 

 name is connected. In the figures to which allusion is now 

 made, attention has been paid not only to angular measurements, 

 but to cleavage-peculiarities. 



We have not been able to form any satisfactory opinion on 

 any of the sections referred to (being unsymmetrical forms), 

 except the one under fig. 20 ; fortunately this is sufficiently 

 decisive for our purpose. The section given in fig. 7, 

 Plate I., is from a drawing made by ourselves, which, it will 

 be seen, closely agrees with Dr. Hull's representation, except 

 that the cleavage is shown a little more in detail. 



It is stated that this is a " section of olivine crystal from the 

 lava of 1794." The mineral or chemical nature of the section 

 we do not dispute ; for it exhibits under polarized light the 

 colours characteristic of peridote, excepting that they are slightly 



* A crystal of augite described by Sandberger lias, disseminated through 

 its entire mass, a substance which he considered to be peridote (Bischof, op. 

 cit. vol. ii. p. 307). We should like to draw Mr. Allport's attention to this 

 case. Moreover the presence of calcite associated with serpentine in Mr. 

 Allport's examples is a noteworthy evidence in favour of our view as to the 

 origin of the " calcareous skeleton" and the calcitic interpolations in the 

 "nummuline wall" of " Eowon Canademe" 



