part 3] LOWER PALEOZOIC of arthog-dolgelley. 275 



total thickness is about 150 feet, but is somewhat variable 

 from place to place. In the lower part of the group occurs a 

 well-marked band of slate 20 feet thick, exhibiting the normal 

 characters of the Bificlus Slates. This slate-band yielded 

 Didymograptus bifidus, along with fragments of extensiform 

 graptolites, at a small trial-level on the western bank of the 

 Crogenen stream. The matrix of the main body of the rocks 

 consists of greenish-grey ashy or gritty particles of a medium 

 degree of coarseness, the constituents having diameters up to 1 or 

 2 mm. The ashy or gritt}^ particles are cemented in a thin 

 paste of argillaceous material, which locally increases considerably 

 in amount. 



These gritty rocks sometimes exhibit, when weathered, a well-marked and 

 regular lamination, and they may also show the effects of cleavage ; the 

 lowest band of grits, below the slate-band, possesses these characters in a 

 marked degree. On the other hand, the lamination may also disappear in the 

 more massive types of grit, while at the same time the rocks yield but little 

 trace of cleavage. Such massive rocks are well jointed, and break down into 

 great rectangular blocks in the same manner as do many of the diabases 

 of the district. In fact, the deceptive resemblance to diabase has evidently 

 misled previous observers on more than one occasion. Another feature that 

 aids in the deception is the frequently-occurring pseudo-vesicular appearance, 

 due to the weathering- out of some of the constituents, leaving hollows which 

 may eventually become infilled with calcite or some other secondary mineral. 



As the gritty series is ascended there is a constantly-increasing tendency 

 for a peculiar lithological type to make its appearance. It consists of argil- 

 laceous material more or less completely silicified, in such a manner that all 

 transitions may be seen from dark, rather hard but otherwise normal, slaty 

 material, to highly- silicified rocks which are harder than steel, possess a 

 splintery fracture, and are closely comparable in appearance with the ' China- 

 stone ' Ashes at the top of the Lower Acid Series. 



If the nature of this material is sufficiently peculiar, its mode of occurrence 

 is even more so. It occurs sporadically distributed in the grits in patches 

 that vary in size from small rounded pellets with diameters of less than 

 a quarter of an inch, up to great irregular lumps and lenticular masses 1 foot 

 thick and 2 or 3 feet long. Usually, the silicified material stands out pro- 

 minently on weathered surfaces, since its smooth white crust shows up in sharp 

 contrast with the rougher and greyer surface of the normal grits. It is this 

 feature that accounts for the characteristic agglomeratic appearance of the 

 beds in which it is found. Occasionally, however, the argillaceous material 

 weathers away, lea.ving a more or less irregular cavity. 



The amount of slaty matter varies rapidly from bed to bed, and even within 

 the limits of a single bed. It may form more than half the total bulk of 

 rock, or it may be absent altogether, or occur sporadically in the grit in 

 isolated and irregularly-distributed masses. As a rule, it may be said that 

 the smaller the masses are the more intense is the silicification, resulting in a 

 rock which, by itself, would be mistaken for a rock derived from a china-stone 

 ash or even from a rhyolite. Certain beds, therefore, that are mainly composed 

 of small ' pebbles ' of silicified material with the ashy matrix quite subordinate 

 in amount, may easily be mistaken for a rhyolitic ash or agglomerate. One 

 such bsd occurs almost at the top of the Bryn Brith Beds, on Bryn Brith 

 itself. 



The larger masses, on the other hand, partake increasingly of the characters 

 of normal mudstones or slates, and in the case of the largest they show the 

 usual cleavage effects. Such large masses tend to be lenticular in shape, 

 the elongation being usually in the direction of bedding, in such a way as 



Q. J. a. S. No. 803. u 



