314 A. GEIKIE ON THE SUPPOSED 



and Mr. Tawney I have but little to add regarding the structure of 

 the mass in its central typical portions. It is a thoroughly crystal- 

 lized compound, with the distinctive micropegmatite structure of a 

 true granite. The quartz is specially abundant in some places, and 

 always presents the characteristic forms of this mineral in granite. 

 The felspars are all more or less kaolinized ; striated forms may be 

 detected among the more predominant orthoclase ; while here and 

 there a little microcline (a species so characteristic of granite) may be 

 observed. In none of my slides have I found any mica ; but in all of 

 them there is an abundant bright grass-green mineral, often in tufts 

 and vermicular aggregates. Most of this green constituent appears to 

 be chlorite. Nests of epidote may also be detected, some of it 

 possibly replacing original mica. The presence of chlorite and 

 epidote, and the turbid condition of the felspars prove the rock to 

 have undergone considerable alteration. 



The microscopic structure of the rock remains tolerably uniform. 

 Towards the margin of the mass, however, the texture is apt to 

 become finer-grained, though this change is not always observable. 

 At Porth-lisky, where the greater closeness of grain in the marginal 

 parts is well seen, portions of the rock assume a structure approach- 

 ing that of graphic granite, and are much veined with calcite. It 

 is here that the bands of sparry carbonates described by Dr. Hicks 

 occur. His so-called " quartz schists " are likewise portions of the 

 graphic condition of the granite*. 



In the course of my study of crystalline rocks in the field, I 

 have never met with the graphic structure except in veins. If the 

 beach could be quite laid bare at Porth-lisky, it is probable that 

 the graphic structure there visible might be found to belong really 

 to veins connected with the main mass of the granite. 



The same graphic structure is well displayed in the veins of 

 lighter, finer-grained granite, or segregation-veins which traverse the 

 main mass in so many places. These veins (known to German 

 geologists by the glass-makers' term " Schlieren," — threads or rib- 

 bons) are quite distinctive of granite, and do not occur among 

 gneisses and schists. Their presence in the rock of St. David's is, 

 in itself, sufficient to prove that rock to be an eruptive granite. 



At the northern end of the ridge the granite is succeeded by 

 masses of quartz porphyry. jSTo continuous section can here be 

 traced ; but there are numerous exposures of rock between Bryn-y- 

 garn and the valley near the cathedral. At Bryn-y-garn itself, the 

 granite appears in its most typical form. A little to the north, at 

 Rock House, what appears to the naked eye as an extremely small- 

 grained granite, approaching to felsite, can be seen. At the bottom 

 of the slope, on the roadside leading south-westwards from St. 

 David's, a rock with still closer texture may be observed. 



A series of thin slices prepared from these rocks leaves no doubt 

 on my mind that there is here a transition from the granite of the 



* Quart. Journ. Geol. Soc. vol. xxxiv. p. 154. Professor Bonney observed 

 the graphic structure in these rocks, though he was disposed to consider them 

 as of ruetamorphic origin. 



