﻿100 



MR. a. W. TYRRELL ON THE 



[vol. lxxii, 



of weathering, calcite begins to appear. Beautiful rosettes of a 

 serpentinous mineral are frequently developed in, and apparently 

 at the expense of, the analcite. These enclose flakes of biotite and 

 also the other minerals usually found within the analcite areas. 

 Occasionally the analcite is completely replaced by the fibrous 

 serpentine. These rosettes have been mentioned by Mr. E. B. Bailey 

 in describing the Glasgow teschenites, and he indicates the need for 

 some special explanation of this phenomenon. He suggests that 

 it may possibly originate from ' juvenile ' reactions. 1 



The analcite corrodes and replaces the felspars enclosing the 

 cavities in which it has crystallized. The attack has usually 

 spread from the cleavage and other cracks, and the process can be 

 followed from mere incipient analcitization, resulting in a widening 

 of the cleavage-fissures, to complete replacement of the crystal. 

 The latter, however, frequently retains its form ; equally often the 

 felspar forms shapeless, irregular masses entirely enclosed in 

 analcite. The final appearance is of a patch of clear or dusty 

 analcite lying in the midst of a large area of partly or wholly 

 analcitized felspars, mixed with serpentinous, chloritic, and other 

 alteration-products. 3 These reactions clearly belong to a juvenile 

 stage in the history of the magma, and may be referred to that 

 late period when the rock was stewing in a hot alkaline solution 

 which ultimately crystallized as analcite. 3 Further evidence as to 

 the original nature of the analcite is afforded by the numerous 

 inclusions of biotite, pyroxenes, apatite, and felspars that it con- 

 tains ; by its association with soda-orthoclase ; and by its evident 

 reaction on the adjacent felspars and augite, resulting in the latter 

 ca,se in the formation of a thin layer of a green soda-pyroxene. 

 Pyroxenes which have been entirely enclosed in the analcite have 

 suffered this change to a much greater extent, leading in some 

 cases to complete replacement. 



It is remarkable that, while a susceptible mineral such as 

 analcite often remains entirely or comparatively fresh, the other 

 constituents of the teschenites have undergone so much alteration. 

 This leads to the conclusion that the alteration is not so much due 

 to ordinary weathering as to the presence, during the crystallization 

 of the rock, of a hot, intensely-active, alkaline, and water-rich 

 mother-liquor, which crystallized as analcite after effecting much 

 corrosion among the earlier-formed constituents. 



Because of the corrosion and replacement effected by the analcite 

 among the earlier constituents of this and other rocks, there is a 

 disposition to regard it as 'secondary' in a certain sense of that term. 

 But it is as definitely a primary consolidation -product of the 

 teschenite magma, inasmuch as its ciystallization took place before 

 the cessation of cooling of the rock, as is the allotriomorphic quartz 

 of a granite. Both represent a final magmatic residuum, which 



1 1 The Geology of the Glasgow District ' Mem. Geol. Surv. Scotland, 1911, 

 p. 132. 



2 ' The Geology of the Neighbourhood of Edintmrgh' ibid. 1910, p. 296. 



3 E. B. Bailey & G. W. Grabham, Geol. Mag. dec. 5, vol. vi (1909) p. 256. 



