﻿628 MR. W. G. EEARNSIDES ON THE GEOLOGY OP [Aug. I905, 



come from anywhere between the Llandeilo and the Bala Lime- 

 stones, The specimens, however, are smaller than is usual at the 

 latter horizon, and forms transitional between pairs of related 

 species are more frequent than forms distinctly referable to those 

 species. The trilobites are not so abundant, and are but rarely well- 

 preserved. Those identifiable are indicative of the higher part of 

 the Lower or of the Middle Caradoc horizon. On the whole, the 

 fauna seems more closely related to that of the various calcareous 

 grits of the East-Shropshire Caradoc Series than to any single series 

 with which I am acquainted : but, in comparing it with those faunas, 

 it is noteworthy that all such large Orthids as Orthis jlabellulum, 

 0. vesjDertilio, etc. are as yet undiscovered. The commonest fossils 

 in the limestone are a rather small form of Orthis biforata, Schloth., 

 0. Actonice, Sow., 0. testudinaria, Dalm., and Plectambonites sericea, 

 Sow. Monticuliporoids, bryozoa, and stems and ossicles of crinoids 

 or cystoids are also exceedingly abundant, and make up a very large 

 proportion of the rock. 



The Dicranograptus- Shales [1]. 



I have not seriously worked out the beds above the limestone, 

 but what I have seen of them does not encourage me to continue. 

 They are an enormous series of soft black shales or slates, with a 

 varying degree of cleavage. They are beautifully exposed along the 

 Nant Hir and other streams which flow down from the dip-slopes 

 of the Upper Ashes to Bala Lake, along clean sections 3 miles or 

 more in length ■ but, although I have passed along these daily for 

 several weeks, I have never been able to note a single outstanding 

 feature which would reveal to me the dip-strike or bedding of the 

 rocks, and, except for some ill-preserved Hartfell Dicranograptidse 

 in Drift-boulders, I have never seen so much as a fossil in them. 



The Intrusions. 



The intrusions of the district belong to two main and two 

 subsidiary petrological types, and must be referred to, at least, two 

 quite distinct periods of eruption. Each occurs in sills spreading 

 along the bedding rather than as dykes, and all are remarkable for 

 the way in which they hold to some fairly-definite stratigraphical, 

 or possibly hydrostatic, horizon right across the district. 



The Hypersthene-Andesites [23]. 



The oldest series of intrusives was erupted at a period contem- 

 poraneous with, or closely subsequent to, the time of the formation of 

 the Lower Ashes and Agglomerates, and like them may be termed 

 hypersthene-andesites. They are volcanic rather than hypa- 

 byssal, amygdular, and probably at one time were only cryptocrys- 

 talline and perhaps glassy. With the Lower Ashes and Agglomerates 

 they have long been known under the name of ' Arenig Porphyry,' 

 and are now being quarried at Arenig Eail way-station as ' Arenig 



