330 Ernest W. Skeats : 



9. — Altered Rocks. 



A. — The Aye and Origin of the Cherts. 



The lig-hter, less silicified rocks seen in many places near the 

 contact of diabase and Ordovician present few difficulties, as 

 they are clearly only moderately altered shales of the Ordovician 

 series. 



A peculiar type of more silicified bedded rocks occurs flanking 

 the isolated diabase of 'J'ranter's paddock, Knowsley East. Another 

 outcrop occurs W. of S. Heathcote railway station, and is also repre- 

 sented among the rocks mapped as Dinesiis Beds. The 8. Heathcote 

 occurrence is mapped with the diabase, the one flankinc; the 

 isolated diabase is mapped as Ordovician. There can be no 

 doubt that all the occurrences represent the same type, while 

 the cherts of Lady's Pass appear to differ only in more complete 

 f ilicification. Their characters can be studied from a section 

 (No. 584) (Plate XV., Fig. 1) of a rock from the flanks 

 of the isolated diabase outcrop, Knowsley East. The rock 

 is bedded and cavernous, and some of the cavities have 

 the shape of felspar crystals, others more elongated sug- 

 gest actinolite. The bedding is only slightly defined in tlie 

 section. Numerous altered crystals of pyroxene occur, and 

 larger fragments of igneous rocks lie in a fine-textured ground- 

 mass now silicified. Tlie rock has all the appearance of a 

 silicified submarine tuff. 



In the case of tlie Black Cherts the problem is more complicated, 

 OAving to the comjjlete niineralogical change which most of the 

 rocks have underg(me. \. rather special type of black chert 

 occurs at the margin of the foliated diabase in Tranter's padd.ick, 

 Knowsley Eaist. It is densie, dark in colour, and not Avell bedded, 

 but less altered stages occur A^-ith it, and show a passage into 

 diabase. 



The more normal well-bedded type of black cherts occurs, 

 among other places, at the N. end of Red Hill, near Gate 47. 

 on the railway line south of the station, near and N.W. of Diorite 

 Knob, near Lady's Pass in Dargile. and at a liill just south-west 

 of Gate 51 on the railway north of the railway station. 



The cherts near Gate 47 junction with the luicro-granite, and 

 with the alluvium, so that their relations with the diabase and 



