032 Mr. Aubrey Strahan [March 17, 



It is of the greatest importance to note that in this coal-field the 

 strata which first cover up the productive series consist of upper coal 

 measures (for the most part devoid of coal), and that a considerable 

 distance has to be traversed before the margin of the Xew Red Sand- 

 stone is reached. The importance h"es in the fact that the upper 

 measures are conformable and parallel to the productive strata. 

 They follow on in an ascertained sequence, and if the horizon of 

 any rock in the sequence is determinable at the outcrop, it becomes 

 possible to estimate the depth to the productive strata at that 

 spot. The Xew, Red Sandstone and the Permian, when present, 

 on the other hand, rest unconformably upon the carboniferous 

 strata, and it is often impossible to predict what will be found 

 below them. 



The upper measures are distinguishable into three groups of 

 strata, differing in general characters, but not so precisely defined 

 as might be desired. The lower group consists mainly of red marls, 

 the middle group mainly of yellow sandstones, and the upper group 

 of red sandstones and marls not easily distinguished from the Xew 

 Red Sandstone. Their characters render it possible for a skilled 

 geologist to recognize the outcrops of these groups, and, provided 

 that he knows their thickness, to deduce the depth to the coal below 

 them. Such work as this has engaged the attention of the Geological 

 Survey, both in Xorth Wales and other parts of the country, for the 

 past few years, and is throwing much light upon the accessibility of 

 some concealed coal-fields which remain to be exploited. 



One of the results of the work in Denbighsliire has been to show 

 that the three groups are repeated Ijy faults — that is to say, that the 

 counteraction of dip by faults which has been proved to exist in the 

 proved extension of the coai-field is likely to exist also in the exten- 

 sion not yet proved. The inference follows that the seams are kept 

 by the faults within an accessible depth for some distance farther 

 than the underground explorations have extended. The details of 

 the faulting, however, are not known, and I have therefore marked 

 this part of the concealed coal-field as " hypothetical." 



Finally, the Xew Red Sandstone comes on and effectually conceals 

 all older formations. Under it the existence of a concealed coal-field 

 is possible, but we have no evidence as yet to show whether or no its 

 thickness is prohibitive. Towards the centre of the Cheshire basin 

 it is certainly great : a lioring near Xorthwich ended in Xew Red 

 Sandstone at a depth of more than 2,500 feet. 



As a third example we may consider the concealed coal-field of 

 Kent. This difi'ers from the other examples in that it is not con- 

 nected with any visible coal-field, but is everywhere concealed by a 

 blanket of Secondary and Tertiary strata, which generally exceeds 

 1,000 feet in thickness. The existence of coal measures under Kent 

 was inferred 'on geological grounds many years ago, but was first 

 proved in 1886 by a boring near Dover."^ Coal was first raised in 



