Yol. 68.] THE GEOLOGY OP MTNYDD-Y-GADEE. 3dS 



we shall reproduce more nearly the conditions when the intrusion 

 occurred. The connexion with the molten magma beneath was 

 through the transgressive limb. The igneous rock came up as a 

 dyke, and then spread as a sill in laccolitic fashion along the beddiDg- 

 planes ; but it did not keep strictly to a constant horizon. 



It should be observed that a similar explanation may possibly 

 apply in the case of the dolerite masses close to Dolgelly. These 

 also, as we showed in our former paper, appear to spread trans- 

 versely to the bedding, and we interpreted them as probably 

 laccolites along an unconformity. But the structure of Mynydd-y- 

 Gader suggests another explanation, and it is in fact by no means 

 impossible that they are the actual continuations of the trans-^ 

 gressive limb of the Mynydd-y-Gader dolerite. 



III. The Petrology of the Igneous Eocks. (S. H. R.) 



(«) The Intrusive Rocks. 



The Dolerite. — Although the dolerite of Mynydd-y-Gader 

 varies much in coarseness, it is singularly uniform in mineralogical 

 character and contains very few accessory minerals. The prevailing 

 type is a fine-grained, rather pale, greenish-black rock, uniform in 

 texture, and containing neither vesicles nor phenocrysts. In rare 

 cases it becomes strongly vesicular, as at certain points on the 

 northern margin of the mass (82) ^ or in narrow sills such as occur 

 to the south of Mynydd-y-Gader. In many places the rock becomes^ 

 very coarse ; but on the whole it is remarkably uniform and fine- 

 grained. At 212 it has a silica percentage of 56'28, and a specific 

 gravity of 2'81.^ It is often strongly columnar, especially in the area 

 lying south of the sources of the Tan-y-gader and Brya-rhug streams, 

 and towards the eastern end of the mass. The columns are often 

 roughly hexagonal, and 2 to 8 feet or even more in diameter. Thfr 

 intrusive character of the junction between the dolerite and the 

 Rhyolitic Series is often very clear, as along the southern border 

 between the south-westerly-projecting tongue (180) and the little 

 pond on the southern margin, where blocks of rhyolite are often 

 included in the dolerite (PI. XXXYI, fig. 2). 



At the spot (254) north-west of the south-westerly-projecting 

 tongue of dolerite (180) the rhyolite has been partly re-fused 

 by the dolerite,^ the resulting rock having a somewhat foliated 

 appearance. Near (180) also veins of a fine-grained pale dolerite 

 (PI. XXXYI, fig. 1) penetrate both the neighbouring rhyolitic 



■■ These numerals in parentheses refer to the numbers marked upon the 

 specimens, and the points at which they were collected are shown by correspond- 

 ing numbers on the map. 



^ All silica-percentages and specific gravities quoted, excepting those on 

 p. 360, were determined in the Chemical Laboratory of University College, 

 Bristol, by Mr.E. M. Lane. 



^ This was pointed out by Mr. Harker, to whom we are much indebted for 

 help in the examination of these rocks. 



