342 Mr. D. Forbes' s Researches in 



These diabases are composed of a felspar base in conjunction 

 with diallage and chlorite, and frequently, owing to incipient 

 decomposition of these silicates, effervesce slightly with acids, 

 thus revealing the presence of a trace of carbonate of lime. 



From the observations of the Geological Survey, it may be con- 

 cluded that the intrusion of these diabases was not of later date 

 than the Silurian period — which result is conformable with what 

 is known as to the age of the appearance of similar diabasic 

 rocks in other parts of Europe*. 



The auriferous quartz lode at Clogau runs about 18° north of 

 east and dips at a high angle (88°) to south, cutting through 

 both the fossiliferous^strata and the intruded diabases ; it must 

 consequently be of later geological age than both of these rocks, 

 and not improbably is even younger than the Silurian formation 

 as a whole ; but at present no data are at hand to fix its exact 

 age with any certainty. 



The mining explorations carried on hitherto appear to indi- 

 cate that this quartz lode is much richer in gold at the parts 

 where it cuts through the Lower Silurian Lingula-beds,with their 

 accompanying intrusive diabases, than in greater depth where it 

 traverses the Cambrian grits. 



The study of mineral lodes leads to the conclusion that their 

 productiveness is in general much influenced by the nature of 

 the strata through which they cut (by the " country," in miners' 

 terminology); and although at present no satisfactory explana- 

 tion has been brought forward to account for it, still it is gene- 

 rally admitted that hard siliceous strata (like the Cambrian grits) 

 are, as a rule, unfavourable in this respect; it is inferred, there- 

 fore, that the superior productiveness of this lode in the Lingula- 

 beds is due to this cause, and not to the accidental presence of 

 the associated diabases in this precise locality, or, in other words, 

 that the lode might be expected to prove equally rich in gold in 



*• The term greenstone appears to be used by the Geological Survey 

 quite indiscriminately, without reference to any distinction of geological 

 age or mineralogical composition, as, for example, the above-mentioned 

 diabases of Silurian age are in no ways distinguished on the map from the 

 postcarboniferous dolerites equally common in Wales. 



The very characteristic rock-species diabase does not even seem to have 

 been recognized by the Survey, notwithstanding that it would be probably 

 impossible to find better opportunities of studying it in all its variations of 

 texture, crystallization, and mineral composition than in the immediate vi- 

 cinity of Dolgelly. The coarse-grained varieties, with the felspar especially 

 prominent, are quarried on the Towan road just beyond the town ; the more 

 chloride ones are well seen in the rocks between Llaneltyd and Tyn-y-Groes; 

 calc-diabase occurs in the mountain-side immediately above Pemnaen Pool; 

 the schistose, close-grained, and compact varieties occur everywhere ; and, 

 lastly, the magnificent Uralite porphyry, as it has been termed, forms hills 

 alongside of the river Mawddach before coming to Tyddynglwadis. 



