372 ME. W. W. WATTS ON THE OCCURRENCE [Aug. 1 894, 



perlitic aspect, which are clearly the result of contraction. Of these, 

 one at the top passes from the quartz into the matrix, and then back 

 again into the quartz. 



Similar features, with some amount of variation, are visible in 

 figs. 3 and 4 (p. 371), in both of which one or other of the cracks may 

 be seen to traverse the matrix for part of its course. Inside a quartz- 

 grain in fig. 4 there are perlitic cracks in a glass inclusion, while in 

 another quartz from the same slide there is a tendency for the 

 cracks to aggregate in the neighbourhood of a matrix inclusion. 



Slide 1 926 displays the difference in the behaviour of quartz and 

 felspar under the cooling strain, the only cracks visible in the latter 

 being those due to cleavage, while the practical absence of cleavage 

 in quartz allows of the formation of spherical cracks. In PI. XVIII. 

 fig. 3 will be seen excellent polygonal cracks in quartz and matrix, 

 with imperfect perlites in both substances. 



It might have been supposed that the independent contraction of 

 the quartz was responsible for the perlitic structure, and that it 

 was quite apart from the generation of perlites in the matrix. A 

 minute inspection of these figures will, however, show that occa- 

 sionally some one crack or other penetrates from the quartz into the 

 matrix, and may even return into the quartz ; and an examination 

 of the thinnest slice with very high powers shows many clear cases 

 to prove that the glass and quartz must have shrunk and cracked 

 together. 



Fig. 1 represents a portion of the slide I 927 drawn with a 1- 

 inch objective, but with the details put in from a lUinch. It is 

 perfectly clear that the quartz is perlitic, and that there are at least 

 four, and probably two more, cases of continuity of crack from 

 quartz to matrix. There is generally a slight deviation in direction 

 when a crack passes from one substance to the other ; but this is 

 very small, and the main trend of the crack is preserved. The 

 drawing also represents diagrammatically the wisps and curls of the 

 trichites and the deposit on the cracks, while it rouses a suspicion 

 that the cracks have slightly faulted the edge of the quartz. 



A more important case is presented in fig. 5 (p. 371), where the 

 quartz is the focus of the perlite and is traversed by two of its cracks, 

 at least one of which is continued for two thirds of the distance round 

 the crystal in the matrix, although the actual continuity is obscured 

 by a slipping of the section during grinding. Somewhat similar and 

 perhaps clearer relations are displayed by fig. 6 (p. 371), in which the 

 quartz is again at the centre of the glass-perlite and is surrounded 

 by a crack which encroaches on the border of a neighbouring quartz- 

 crystal ; while in PI. XVIII. fig. 2 a crystal of quartz is cut by two 

 perlitic cracks, each of which traverses the matrix as well, both 

 being included in a single circular crack, which does not touch the 

 quartz. In PI. XVIII. fig. 1, again, the quartz is the focus of a 

 perlite which is completed in the matrix ; fig. 1 a shows the outline 

 of the quartz in this figure. 



Two more important cases remain for description. In PI. XVIII. 



