A. Heard — The Petrology of the Pennant Series. 87 



a tarnished brown to black. The mineral was picked out by means 

 of a hand magnet, and was decomposed in hydrochloric acid with 

 an evolution of gas. 



Chlorite occurred as green scaly aggregates, and also as well- 

 defined green pleochroic cleavage-flakes. This was one of the 

 most abundant of the heavy minerals. 



Fluorite has only been observed in one separation from the middle 

 of the Pennants in the South Crop, south of Caerphilly, in the form 

 of triangular cleavage flakes, resulting from the octahedral cleavage. 

 These flakes were colourless and isotropic, and had a refractive 

 index less than that of Canada Balsam. 



Corundum was detected occasionally in the crops of the minerals 

 from the sandstones, but a much larger proj)ortion exists in the 

 shales. It occurs as beautiful patchy royal-blue crystals with a very 

 high refractive index and Ioav polarization colours. These crystals 

 appear to have a prismatic form, Avith cross striations. 



Pyrites occurred in irregular nodules, with a bronze-yellow colour 

 in reflected light ; these nodules were insoluble in hydrochloric 

 acid, but soluble in nitric acid. 



Biotite. — A few thin plates of this mineral have been observed, 

 varying in colour from a dark brown to black. These flakes were 

 drawn from a crop of density greater than 2'9 and less than 3*16 

 by means of the electro-magnet. 



Glauconite. — A few slightly magnetic grains of density less than 

 2'6 have been referred to this mineral. 



Tourmaline in yellowish-brown grains is fairly common. 

 There is a marked pleochroism in these brown grains. Frequently 

 these grains appear as worn, elongated prisms with a high 

 birefringence. 



A few grains of a blue mineral occurring in rounded fragments 

 with a very slight pleochroism have been identified as blue 

 tourmaline. These grains appear to be distinct from those described 

 above as corundum. 



The Cement. 



From the beginning of the work considerable difficulty was 

 experienced in determining the nature of the cement, which in thin 

 section appeared merely as a blotchy aggregate. White mica and 

 silica were the only minerals which could be identified with certainty. 

 The remarkable clearness of the grains of quartz and the absence 

 of a reddish colouring precluded the presence of iron oxide ; and 

 although both siliceous and carbonaceous materials were present, 

 these formed only a small proportion of the cement. 



When the powdered Pennant Sandstone was treated with hydro- 

 chloric acid a brisk effervescence immediately began, and increased 

 on warming. The lime-water test proved this gas to be carbon 

 dioxide. The solution on testing revealed the presence of both lime 

 and iron, and the difficulty arose of proving that the iron, as well as 

 the calcium, was originallv in the form of a carbonate. After 



