Alfred Brammall — Tlie Genesis of Chiastolite. 225 



Normal Alteration Effects. — Though the alteration produced in the 

 shales by the intrusive sheets is of the low grade and small extent 

 usual for such intrusions, the effects include features hitherto recorded 

 as occurring only within the contact zone around acid intrusives. 



Shale taken from an horizon about 5 feet above the upper limit of 

 the higher sheet shows no certain evidence of thermometamorphism 

 beyond induration. Within the alteration zone proper (varying from 

 3 inches to 2 feet in vertical depth) the progressive alteration is as 

 follows : — 



1. Normal black shale, without pyrites. 



2. Harder, fissile ' card ' or ' biscuit ' shale, with pyrites, developed mainly 



along the planes of separation. 



3. A lighter- coloured, more compact rock : the shale has been partly 



bleached by the dissipation of the carbon content ; the fissile 

 character is less pronounced, the traces of the separation planes 

 being pencilled out by pyrites. 



4. A greyish, compact, sub-porcellanous rock, having the appearance of 



adinole. 



The microscopic appearance of the two end terms of this series is 

 ■described in the Appendix. 



A suspected occurrence of Embryonic Chiasiolite. — Of exceptional 

 interest, however, are features observable in the shales which have 

 suffered alteration either (1) as xenolithic slabs enclosed in the 

 thicker sheets of igneous rock or (2) as the platform forming the 

 floor of a sheet. Thin sections from the non-porcellanous zone of 

 card-shale have the following features : — 



1. The base is a pronounced reddish-yellow. 



2. Granular pyrites is extremely abundant, as spheroidal aggregates 

 with radial structure disposed mainly along the bedding planes. 



3. Opaque (? carbonaceous) matter is present in considerable 

 quantity. 



4. The base has suffered a partial recrystallization, of an 

 unexpected character. It is largely made up of a mineral giving 

 rhombic, square, and polygonal sections, the boundaries of which are 

 neatly pencilled out with dark opaque matter — possibly the excretive 

 effect of crystallization, for the outer zone of each section is 

 comparatively free from included opaque matter which, however, is 

 frequently massed at the centre and less frequently at the corners or 

 disposed along diagonals. These sections give aggregate polarization 

 (1st order greys and yellows); hence, neither refractive index nor 

 interference figure nor sign of birefringence is determinable for 

 the original mineral, which has obviously paramorphed or been 

 pseudomorphed. 



No indication whatever of the crystal content of these altered shales 

 is visible in the hand-specimen ; yet it is- difficult to avoid the 

 conclusion that the crystal sections in question represent embryonic 

 imperfectly individualized chiastolite, which by increment of alkali or 

 possibly lime has been pseudomorphed to a schimmer aggregate. 



Discussion of the Genesis of Chiastolite. — If this conclusion be 

 correct, the occurrence of chiastolite is less restricted than is at 

 present supposed. 



DECADE VI. — VOL. II. — NO. V. 15 



