Correspondence. 95 



deposits formed a single series, which could be subdivided upon stratigraphical 

 grounds. But although there was no evidence of unconformity between the strata, 

 he thought that the fact of different groups of fossils succeeding each other in the 

 same area showed that those groups existed in neighbouring seas, and had been 

 driven by upheaval of the sea-bottom on which they lived, into the region in which 

 they are found. Hence he maintained that a change in the forms of life is evidence 

 of unconformity in an adjoining area. 



Mr. Maw remarked that under the Cambrian rocks at Llanberis there are un- 

 conformable beds, which may be the equivalents of the so-called Greenstones of St. 

 David's. 



Mr. Hicks admitted that the subdivisions at present in use may need to be 

 modified. He thought that the greatest break is between the Menevian and the 

 Lingula Flags, few species passing from one to the other. He regarded the upper 

 and lower portions of the Tremadoc as really distinct. 



OOZEfcZE&IESIF'OICsriEmJIN'CIE]. 



VOLCANIC EOCKS OF THE LAKE-COUNTRY. 

 Sir, — As much interest attaches to the question of the relation of 

 the volcanic rocks of the Lake-country (the Green Slate and Porphyry 

 series) to the older Skiddaw Slates, and as one of us formerly 

 expressed an opinion on the subject contrary to what the subsequent 

 researches of the Geological Survey have made out, it may interest 

 some of your readers to learn that we have lately discovered in 

 Swindale, near Shap, beds of volcanic ash of the Green Slate and 

 Porpbyry series clearly interbedded with the Skiddaw Slates, similar 

 to tbe case discovered by Mr. Aveline near Black Comb. 



J. E. Dakyns, Kendal, 



J. Clifton Ward, Keswick. 



DEEP BORING IN PRUSSIA. 



Sir, — I send you some particulars, with which I have lately been 

 favoured by Professor A. von Koenen, of Marburg, respecting tbe 

 deep boring made by the Prussian Government Engineers at Speren- 

 berg, about 25 miles south of Berlin, and noticed by your cor- 

 respondent J. P. at page 48 of the Geological Magazine for 

 January. 



Tbe boring for the first 956 feet (1297|- English feet) was made 

 by manual labour, at a cost of about £1600. 



Several accidents having happened, the borehole was then lined 

 for a depth of 85 feet (115^ English feet) with tubes of 15 incbes 

 diameter ; beyond that, to the depth of 100 feet (135 English feet), 

 with 14-inch tubes ; and then to 363^ feet (493^ English feet) with 

 tubes of 12^ inches diameter. 



The length of time occupied in the above-mentioned work was 

 fifteen months — comprised between May, 1867, and July, 1868. 



From the depth of 956 feet (1297^ English feet), for the remain- 

 ing distance, the boring was carried on by means of a steam-engine. 

 The length of time consumed in sinking this additional 3095 feet 

 (4255 feet English), comprised between January, 1869, and the 

 15th September, 1871, was about 31^ months, or 2 years and 7 

 months ; during which interval several accidents occurred. 



