188 Correspondence — Prof. G. A. J. Cole — A. StraJian. 



magnesian limestone, and that the 5 to 15 per cent, of magnesium 

 carbonate contained in them is a mere impurity, when compared 

 with the 30 per cent, in the matrix from which they have arisen. 

 It is interesting to come across a similar statement made in 

 1817, though we waited long for Professor Garwood's numerical 

 proofs, and for a complete account of the mode of origin of the 

 concretions. Mr. N. J. Winch (Transactions of the Geological 

 Society of London, vol. iv, p. 9) remarks that "botryoidal masses of 

 fetid limestone devoid of magnesia, in balls varying from the size 

 of a pea to two feet in diameter, imbedded in a soft, marly, 

 magnesian limestone, are found at Hartlepool, etc." Winch had 

 given a specimen some twelve years before to James Sowerby 

 (" British Mineralogy," table 38), and the passage above quoted 

 was incorporated by Conybeare & Phillips in their " Geology of 

 England and Wales," 1822, p. 306. Grenville A. J. Cole. 



Dublin, 3[arch 1, 1901. 



SUCCESSION OF STRATA IN THE YOREDALE ROCKS. 

 Sir, — Mr. Dakyns is right in his criticism on the succession 

 I quoted for the Yoredale strata of the Yore "Valley. It is true that 

 the sequence, though there are many exceptions, is usually — 

 Shale. 

 Limestone. 

 Sandstone. 

 But this may be put in another way. The series as a whole is 

 made up of repetitions of this threefold cycle, and may with equal 

 correctness be regarded as consisting of repetitions of the cycle — 

 Sandstone. 

 Shale. 

 Limestone. 

 We have, therefore, the same evidence of intermittent and more or 

 less rhythmic sedimentation which I claimed for the Coal-measures. 

 But there is this difference, that whereas in the Yoredales the cycle 

 commences with inactivity (limestone) and proceeds to rapid 

 sedimentation (sandstone), in the Coal-measures it commences with 

 activity (sandstones and conglomerates) and proceeds to stagnation 

 (coal-seams), the order being — 

 Coal. 

 Shale. 



Sandstone and conglomerate. 

 Both formations result from rapid sedimentation over a subsiding 

 area, but whereas the Coal-measures are essentially estuarine, the 

 Yoredale rocks of the type developed in the Yore Valley bear every 

 sign of having been laid down in open sea ; the one was a product 

 of the shallowest water, the other of comparatively deep water. 

 Herein probably lies the explanation of the reversal of order of 

 events. 



I am obliged to Mr. Dakyns for the correction. 



A. Strahan. 

 March 6, 1901. 



